The apparent effects of environmental factors on skin structure in hatchery-reared brown trout are mediated by the influence of these factors on the skin parasite population. The presence of ectoparasites [Trichodina, Scyphidia, Gyrodactylus, lchthyobodo and Saprolegnia] promotes a reduction in the concentration of epidermal mucus-secreting goblet cells, the magnitude of which is related to the intensity of infection.