Castelli Core Mesh Baselayer
Even on the hottest days wearing a base layer is always a good idea. The more sweat that's pulled off your body the more comfortable you will be. Castelli's Core mesh base layer is made from hollow core fiber for maximum wicking ability while using a minimal amount of material.
For best performance you want the Core base layer to fit as snug as possible. To us that would mean selecting your regular size and not your "Castelli size" as their jerseys have a history of running a little small. For example if you typically wear a medium but wear a large a Castelli jersey, order a medium base layer.
Lastly, we're not big fans of bike snobbery and telling folks what or what not to wear but trust us when we say we don't recommend wearing the Core base layer by itself, unless of course you're deliberately trying to pull of the "bad guy from Urban Cowboy look," then go for it.