Reference ranges for serum osmolality and 22 serum constituents were determined for channel catfish (Ictalurus punctatus) reared under commercial conditions in Mississippi ponds for 8 months. The constituents analyzed were sodium, potassium, chloride, calcium, magnesium, inorganic phosphate, urea nitrogen, uric acid, creatinine, glucose, cholesterol, triglycerides, total protein, albumin, total bilirubin, lactate dehydrogenase, γ-glutamyltransferase, aspartate aminotransferase, alanine aminotransferase, creatine transferase, alkaline phosphatase, and α-amylase. Constituent concentrations varied markedly among individual fish, but were unaffected by stocking densities ranging from 9,800 to 19,800 fish/hectare. Glucose and potassium concentrations declined significantly during winter months. Creatinine kinase concentrations were affected by the amount of muscle activity and were especially high in fish that had suffered trauma from other individuals.