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Castelli Seamless Men's Sleeveless Base Layer


Item # CST2600

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  • White, S/M ($49.95)
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Item # CST2600

It's not cool to be hot.

If it's hot out, and you're not wearing a base layer, you're doing it wrong. A proper base layer, like the Castelli Seamless Sleeveless Base Layer, will efficiently manage moisture and actually keep you cooler.

To achieve the aforementioned, Castelli constructed the base layer from two fabrics, polypropylene and nylon. As you might imagine, the materials express a duality to the base layer's function -- pulling and drying. Accordingly, the inner-layer is comprised of the polypropylene material. This fabric effectively pulls moisture away from the skin, transferring it to the outer-layer of nylon. Why nylon? Because its lightweight, airy composition is an efficient evaporator, drying quicker than most materials. So, instead of simply soaking in your sweat, the base layer keeps you cooler by evaporating moisture next to the skin. This has been furthered by the three different mesh patterns on the base layer. Taking anatomy into account, this system maintains a consistently open mesh in order to provide the most ventilation possible. Accordingly, this keeps you cooler, as well. Don't believe us? It's time for an experiment. Blow on the back of your hand while dry, and now blow on the same spot after applying a minimal amount of moisture? Cooler right? Well, that's basically the principle behind this base layer.

The Castelli Seamless Sleeveless Base Layer is available in the color White and in the sizes Small/medium, Larg/x-large, and XX-Large.

Tech Specs

[inner layer] polypropylene, [outer layer] nylon
Claimed Weight:
[size large] 57 g
Recommended Use:
Manufacturer Warranty:
1 year

Size chart


Men's Apparel
 Small Medium Large X-Large 2X-Large 3X-Large
Chest  35.5 - 37in
(90.2 - 94.0cm)
37 - 38.5in
(94.0 - 97.8cm)
39 - 41in
(99.1 - 104.1cm)
41 - 43in
(104.1 - 109.2cm)
43 - 45in
(109.2 - 114.3cm)
45 - 47in
(114.3 - 119.4cm)
Waist 29.5 - 31.5in
(74.9 - 80.0cm)
31 - 33in
(78.7 - 83.8cm)
33 - 35.5in
(83.8 - 90.2cm)
35.5 - 37.5in
(90.2 - 95.3cm)
38.5 - 40.5in
(97.8 - 102.9cm)
41 - 43in
(104.1 - 109.2cm)
Hips 36.5 - 38.5in
(92.7 - 97.8cm)
38 - 40in
(96.5 - 101.6cm)
40 - 42in
(101.6 - 106.7cm)
42 - 44in
(106.7 - 111.8cm)
44 - 46in
(111.8 - 116.8cm)
46 - 48in
(116.8 - 121.9cm)
How to Measure:
For best results, take measurements over your underwear.
Measure under your arms, around the fullest part of your chest.
Measure around your natural waistline, keeping the tape a bit loose.
Measure around the fullest part of your body at the top of your legs.
Using pants that fit well, measure the crotch seam to the bottom of the leg.

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Here's what others have to say...


fits tights and feels great

  • Familiarity:I've used it several times
  • Fit:True to size

This is what an undergarment should be: comfortable, provides breathing space and tight enough not to bunch up under the rest of the kit. Due to the tight fit, if you've got shoulder issues, you may need help getting it off. I find it wonderful.

Avg. ride time: 2h 29m per week
  • Average ride time is based on Strava activity over the last 3 months. Give your reviews credibility by connecting your account.

Work great!

  • Familiarity:I've put it through the wringer
  • Fit:True to size

I got this base layer for the summer rides and it performs great. What i discovered is that it works even when the temperature goes down. I have used it with my long sleeve jersey and a vest in the low 40's. I found that i am actually warmer with the base layer than without. Reason, my thinking is that i am putting space (air is a wonderful insulator) between my body and jersey.
Works great in the summer too, but that is what it is for.

Avg. ride time: 4h 58m per week
  • Average ride time is based on Strava activity over the last 3 months. Give your reviews credibility by connecting your account.

My favorite base layer!

  • Familiarity:I've put it through the wringer
  • Fit:True to size

I absolutely love this product. It is my go-to base layer, so much so I feel naked without it. I'm hoping santa brings me a few more for Christmas this year. It runs true to size, you definitely want it to fit snugly in order to function properly. Once you wear it, you'll never want to ride without it.


Yes, a must in the summer!

    This summer vest is designed to keep you cool in warm and hot weather by holding your jersey *off* your skin. In addition, the special hollow-fiber textile pulls sweat off your body and evaporates it away. It works!

    Every serious rider should consider owning a garment like this for riding in warm weather. This vest baselayer combined with a good summer jersey make a great combination. You will run cooler!

    FIT: I'm 5'10" and 185. I usually wear a L in street clothes. And an XL in Castelli. The L/XL fit great (snug and wrap around, as designed), but was a bit long on me. So I turn up the fabric at the bottom. The size L/XL will certainly fit tall riders. Up to 6'2", I would think.


    Very cool

    • Familiarity:I've put it through the wringer
    • Fit:True to size

    Very confortable, it didnt stink as other baselayers

    Very cool

    I'm right between the medium and large; what size should I order? I normally wear medium jerseys, but the occasional large for euro brands. The small being paired with the medium has me worried!

    Best Answer

    I typically wear M jerseys as well. I'm 6' 160lbs and the S/M fits great. The mesh material can stretch a lot and the length is good.

    Avg. ride time: 2h 41m per week
    • Average ride time is based on Strava activity over the last 3 months. Give your reviews credibility by connecting your account.

    Great base layer

    • Familiarity:I've used it several times

    Castelli's Seamless base layer is great! I am 5'8 - 5'9 165lbs and I have a stocky short torso.

    The base layer is very true to size for Castelli's line of cycling gear. It sits well around the neck, sleeves and hip area. Its snug enough to wear over my jerseys without discomfort.
    The material is fairly thicker than the base layers I am familiar with but even in summer Southern Californias Summer weather the layer does it jobs with ease.
    It feels very sturdy even through all the washes it has endured so I am very pleased.

    Sorry for the late reply. I bought a Medium sized base layer!


    ??? ??????.

      이번에 카스텔리에서 Sleeveless Base Layer를 구입했는데, 착용감이 너무 좋아 벌써 부터 여름이 기다려집니다.

      절대 강력 추천하는 품목입니다. ^^

      Does this base layer provide sun protection?...

      Does this base layer provide sun protection? i.e. can I use it under the Climbing jersey and not worry about getting a sunburn?

      Hi Perry,

      The Castelli Seamless Men's Sleeveless Base Layer would not provide adequate sun protection.

      It has a very open mesh weave that would expose the skin to the sun.


      Must have in summer

      • Familiarity:I've put it through the wringer

      This base layer is like having air conditioning in the summer! It creates space between your body and your jersey to allow air to flow in between and cool your skin. It also pulls the sweat off your body. Makes a big difference when riding in 90-100 degree temps. I am 6'3", 195 lbs and went with the L/XL as the piece stretches and you want it to fit tight.

      B7, which is better? Castelli's "seamless" baselayer above... or Castelli's "core mesh" vest baselayer? They seem almost identical.

      According to CC, the seamless has different mesh patterns to breath better in key areas.

      I own both the seamless and core mesh and prefer the seamless on hot days. The seamless is thicker, but that allows more air to pass through between your jersey and your skin. The core mesh still does it's job, but the seamless is a few notches above.

      Avg. ride time: 8h 2m per week
      • Average ride time is based on Strava activity over the last 3 months. Give your reviews credibility by connecting your account.

      A better view of the baselayer

      As you can see from the open mesh, it's nothing like a t-shirt baselayer. Looks pro with a jersey, though.

      A better view of the baselayer
      Avg. ride time: 8h 2m per week
      • Average ride time is based on Strava activity over the last 3 months. Give your reviews credibility by connecting your account.

      This baselayer won't overheat you

        I never would have thought that a baselayer would be a good thing to have, but a crash where I got road rash on my back showed me otherwise. An extra layer might have spared me a week of road rash healing.

        But...a baselayer for hot weather? Crazy! But not really. The mesh construction lets air flow freely, helping keep me cool. Most telling, my torso feels reasonably dry during the ride but when I take the jersey off it's soaked. So the baselayer transfers sweat outward.

        The material's stretchy, so as small as these are they're more forgiving than they look. But don't wear these minus the jersey unless you want weird looks.