1. Channel catfish (Ictalurus punctatus) take up zinc (measured with 65Zn) from external ambient solutions in a concentration dependent manner. At a concentration of 10(-6)M Zn, this uptake is equivalent to 0.4% of the total body Zn each day. 2. Zinc influx was increased by external acid conditions (decreasing pH from 7.3 to 5). 3. Elevated Ca2+ and Cd2+, but not Al3+, concentrations markedly decreased the uptake of Zn. 4. These observations may be relevant to circumstances that occur under natural conditions, and influence the zinc nutrition and toxicity of the fish.