Water quality changes during establishment and maintenance of nitrification in four 70-liter recirculating systems each containing 20 channel catfish (Ictalurus punctatus) (total weight 150 g) are described. Establishment of nitrification took 33 days. During establishment of nitrification, all catfish in the systems died. Death appeared to be related to nitrite concentrations greater than 2.5 mg/liter NO2-N. The pH declined with progressive nitrification. When the pH was less than 6.0, nitrification ceased. Within 2 wk of introducing the catfish, total bacterial counts stabilized at 106 organisms per milliliter. Counts of lactose fermenters and aeromonads fluctuated around 104 organisms per milliliter when fish were in the system and decreased markedly after the fish were removed. After establishment of nitrification, 40 channel catfish (total weight 100 g) were maintained successfully for 37 days.