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Castelli Seamless Base Layer - Short-Sleeve - Men's

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Item # CST2599

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Item # CST2599

Layer up.

During rides that involve low temperatures, your base layer acts as an insulator for your body. Similarly, in warm weather, that same layer under your jersey works to pull moisture away from your skin in order to keep you cool and comfortable. Castelli understands that the inconspicuous under-layer is hard at work regardless of your riding weather, which is why it introduced the Seamless Base Layer.

Castelli's Seamless Base Layer was constructed using two fabrics: polypropylene and nylon. These materials work in tandem in oder to perform the base layer's primary function -- pulling moisture away from the skin and drying it. Accordingly, the inner-layer is comprised of the polypropylene material, as it effectively pulls moisture away from the skin and transfers it to the outer-layer of nylon. And because nylon is lightweight and maintains an airy composition, it's a naturally efficient evaporator. The material dries much quicker than most, which means that instead of simply soaking you in your sweat, the base layer keeps you cooler by evaporating moisture next to the skin.

This moisture transfer is 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, which keeps your body dry and comfortable throughout your rides.

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

Tech Specs

[inner-layer] polypropylene, [outer-layer] nylon
Recommended Use:
Manufacturer Warranty:
30 days

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...


Great baselayer for warm temps.

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

A great base layer for warm to hot days. It is well made and comfortable -- and creates a bit higher layer than the similar mesh base from Castelli. I like how this piece prevents chafing and keeps me cool. It is well made too and holds up in the wash, though I usually use a garment bag to keep it safe.

Avg. ride time: 7h 34m per week
  • Average ride time is based on Strava activity over the last 3 months. Give your reviews credibility by connecting your account.

Makes all the difference

  • Familiarity:I've used it several times

I was skeptical that a baselayer could be used effectively under a jersey in both cold and dry temperatures, and I am so happy that this skepticism was wiped away. This undershirt keeps the jersey off of your skin and wicks away sweat so well that you never get that "wet jersey" feeling on those long hot rides. On the other side, when it's cold outside, it keeps that layer of warm air close to you thanks to the "unique" fishnet design. I will not be riding without this and may buy a second at this price point.


Buy this

    Castelli offers arguably the largest lineup of pro & performance cycling clothing made in the sport. The experience, detail, and 'know-how' that Casteli puts into their gear is pretty amazing. This is seen for example in Castelli's lightness glove, climber's jersey, Mortirolo jacket, and even their cycling socks.

    This "seamless" baselayer is designed to be used in -warm- weather. It is made with special moisture transfer fabrics. It is also designed with "open mesh" construction to hold your jersey off your skin. These features promote moisture transfer, sweat evaporation and cooling. It works!

    Every serious rider should consider using a baselayer like this in warm weather.


    A micro-climate for your core!

    • Familiarity:I've used it once or twice and have initial impressions

    I'm a big believer in technical undershirts for low temperatures on up to about 80 degrees F. Beyond 80 in a low humidity environment I can't see much benefit over having a good quality jersey against my skin, but there are situations where it could still be helpful. This undershirt is perfect for the temps I've been riding in lately, 45 to 65 degrees. It creates a nice layer of trapped air while efficiently moving moisture away and keeping me dry with a stable core temp. It's very mesh like, and given it's stretch the sizing is very forgiving. It has a nice long torso and lays flat against your skin. It will look very small when you pull it out of the box. My wife laughed when she saw it and insisted I never let anyone see me in it! So of course I made it a point to walk around the house in it! My only complaint is that the material will snag easily. Use caution if you're wearing a watch when you put it on, and perhaps, be willing to accept that you'll get some snags that widen some of the mesh openings over time.


    Totally Awesome Base Layer!

    • Familiarity:I've put it through the wringer

    Unbelievable fabric & mesh design. Wicks better than any other base I've used in the past. Ideal for hot days too! It keeps a dry (not clammy) layer against your skin thus keeping you comfortable on hot days. Wicks moisture to outer jersey where it evaporates. I love this base. I am 5'7" @ 165lbs. chest 40" and a lg/ex lg. size fits snug like it was designed to fit for maximum efficiency.

    Avg. ride time: 6h 13m per week
    • Average ride time is based on Strava activity over the last 3 months. Give your reviews credibility by connecting your account.

    awesome base layer!

    • Familiarity:I've put it through the wringer

    The large seem small but stretches for a nice fit. it is very comfortable and warm but not hot. The mesh is like magic and keeps me dry.

    Avg. ride time: 5h 4m per week
    • Average ride time is based on Strava activity over the last 3 months. Give your reviews credibility by connecting your account.

    Awesome Base Layer

    • Familiarity:I've used it once or twice and have initial impressions

    This base layer did exactly what I was hoping it would do. It helps me maintain an even body temperature in various Fall climates and keeps my jersey from getting sweaty and cold against my skin. It works well for me in temps from mid 50 degrees to mid 80 degrees, but I have hopes that I can use this base layer year round. Very impressed with the quality as well. I can barely tell it's on when I wear it. One note, when you get first pull it out of the box it will be tiny like a child's shirt. I'm 5'10' 150lbs. and the S/M fits perfect.