Histological evidence failed to confirm that ammonia is a direct causative agent of gill hyperplasia, as previously believed, in channel catfish, Ictalurus punctatus. Levels of residual chlorine compounds, which are typical of charcoal-filtered municipal water or chlorinated sewage effluent, together with moderate ammonia levels caused severe gill hyperplasia. These results both question the validity of attributing gill tissue hyperplasia to ammonia alone by past investigations and emphasize the serious problem of obtaining contaminant-free water for aquatic toxicological studies.Key words: Ictalurus punctatus, channel catfish; ammonia, gills, chloramine, hyperplasia, water quality