The effects of acid water on alcian blue-periodic acid Schiff's stainable mucosubstances of epidermis from the anterior abdominal wall of brown bullhead catfish, Ictalurus nebulosus, were studied. Standard morphometric techniques were used to determine mucous cell volume density (MCVv) and number density (MCNv) of control (pH 6.8-7.0) and acid-stressed (pH 5.7-6.15) fish. Mucous cell volume density was significantly increased (P less than 0.005) in fish which had been exposed to acid-stress for five days. Since increase in volume density can arise from either an increase in the size of individual mucous cells (hypertrophy), an increase in the number of mucous cell per unit volume (hyperplasia), or a combination of the two, average cell data were computed. No significant difference in cell diameter or cell volume between treated and control groups was seen. However, the number of mucous cells in acid-stressed fish was nearly twice (p less than 0.005) that of controls. These results demonstrated that acid-stress resulted in increased number but not volume of mucous cells (i.e., hyperplasia without hypertrophy).