The effect of captivity on the epidermis of Pseudopleuronectes americanus Walbaum is influenced by the phase of the reproductive cycle. Towards the end of the prespawning period captive male flounder had a higher GSI than flounder from the wild population, and the seasonal epidermal sexual dimorphism in these captive flounder was more pronounced. Flounder maintained in the laboratory during the postspawning period manifested a thick epidermis and increased mucigenesis in late summer, when these characteristics displayed seasonally low values in the inshore population from which the flounder were sampled. Possible regulatory factors and implications for protocols involving captive fish are discussed.