1. Temperatures of 18 degrees C for acclimation or assay had minimal or no effect on channel catfish phagocyte function. Significant suppression was observed at 10 degrees C acclimation and assay temperature. 2. According to the results of a multiple acclimation/assay temperature combination study, the primary impact of temperature on phagocyte function was due to the assay temperature. 3. The only functional change caused by acclimation temperature was the possible adaptation of the respiratory burst. However, 10 degrees C acclimation did cause a decline in the number of lymphocytes in the anterior kidney but not the number of neutrophils. In a temperature-kinetic study, the suppressive effect of 10 degrees C assay temperature was confirmed. 4. Results of our study indicated that phagocytosis in channel catfish is temperature-mediated. However, phagocytes appeared to be more resistant to low temperature than lymphocytes, which implies the importance of phagocytosis in the defense mechanisms of channel catfish at low temperatures.