1. 1. The activities of carbamyl phosphate synthetase, ornithine transcarbamylase, arginine synthetase system and arginase were measured in liver and kidney homogenates of cultured and native channel catfish. 2. 2. Significant levels of carbamyl phosphate synthetase, ornithine transcarbamylase and arginase were detected in liver tissue, whereas only ornithine transcarbamylase and arginase were detected in kidney tissue. 3. 3. The liver and kidney arginases were activated by Mn 2+ ions and exhibited K m values similar to those previously reported for ureotelic species.