Is protein polymorphism adaptive or neutral1? Attempts to assess the neutrality theory on the basis of gene frequencies and theoretical population genetics models seem, at least to some authors, to be unsuccessful2. Possible promising approaches involve the search for direct correlation of isozymes with the environment1 and with physiological function3. We have tested the effects of temperature on barnacles, because temperature affects protein structure and function4,5,6, as well as size7, and because adult barnacles, following larval settlement, are stationary and directly exposed to local temperature variation. Our results suggest strong temperature selection on allozymic and size variations in the acorn barnacle, Balanus amphitrite.