- A nitrogen (N) budget was developed for four, 400-m 2 ponds stocked with 550 channel catfish (Ictalurus punctatus) fingerlings that were fed to satiation daily for 133 days with a ration containing 4.85% N. Feed accounted for 87.9% of the N input to ponds. Abundant N from ammonia (NH3), ammonium (NH4 ), and nitrate (NO3 ) and the high total N: total phosphorus ratio in pond waters prevented appreciable biological N2 fixation. There were four main N losses: fish harvest (31.5%); denitrification (17.4%); NH3 volatilization (12.5%); accumulation in bottom soils (22.6%). Nitrification averaged 70 mg N m 2 d 1 , denitrification averaged 38 mg N m 2 d 1 , and phytoplankton removed NO3 Na t 24 mg Nm 2 d 1 . Mineralization of feed N to NH3 averaged 59 mg N m 2 d 1 . As feed is the largest N input in catfish ponds, improved feeds and feeding practices can increase the proportion of N recovered in fish and reduce the amount of NH3 excreted by fish. Efficient aeration and water circulation also should enhance nitrification and oxidation of organic N. © 2000 Elsevier Science B.V. All rights reserved.