The effects of acid water (sulfuric acid addition to pH 4.0 or 5.7) on epidermal mucosubstances of the brown bullhead (Ictalurus nebulosus) were studied. Volume density of mucous cells was increased over controls in acid stressed fish (187% and 190% respectively). No significant difference in cell diameter or cell volume between treated and control groups were seen. These results coupled with auto-radiographic localization of ³H-thymidine over nuclei in basal and intermediate zones of skin demonstrate a hyperplasia of mucous cells. Sex related differences were seen in treated and control fish although each sex responded to acid stress. Computer assisted morphometry is a valuable aid in establishing the quantitative nature of pollutant-induced alteration in fish tissues.

*Supported in part by the Water Research Institute, West Virginia University, with funds allocated under the Water Resource Act of 1964 (PL 88-379) administered by the Office of Water Research and Technology, US Department of Interior.