The effects of short-term infection with the branchurian crustacean ectoparasite Argulus foliaceus, and the fish stress hormone cortisol (which is reported to stimulate mucus discharge), were studied on the mucous cell population of the head skin of rainbow trout (Oncorhynchus mykiss). Argulus infection did not raise plasma cortisol significantly and had no effect on the number of epidermal mucous cells in the head skin. Cortisol was administered twice to groups of trout via the food, significantly elevating circulating plasma cortisol at 24 h post feeding without affecting numbers of mucous cells, and increasing the numbers of vesicles in the upper cells of the epidermis. Subsequent infection with the parasite (6 Argulus/fish) did not affect either plasma cortisol or total numbers of mucous cells at 48 h post-infection with the parasite, but led to a significantly lower parasite infestation per fish in the cortisol-administered groups. A 24 h culture system was used to expose pieces of trout skin to 50 ng/ml cortisol in vitro to investigate whether cortisol alone would stimulate reductions in mucous cell numbers. These were unaffected by the addition of cortisol. The in vivo and in vitro results are discussed in relation to the current understanding of crustacean host-parasite interactions.