The rapid development of aquaculture could result in significant environmental concerns such as eutrophication and climate change. However, to date, very few studies have been conducted to investigate nitrogen transformations in aquaculture systems; and specifically the emission of nitrous oxide (N2O), which is an important greenhouse gas and ozone-depleting substance. In this study, nitrogen transformations in intensive laboratory-scale Chinese catfish (Clarias fuscus) aquaculture systems were investigated by identifying and quantifying N2O emissions. Results indicated that about 1.3% of the nitrogen input was emitted as N2O gas. Dissolved oxygen (DO) concentrations and feeding rates had significant effects on N2O emissions. Higher N2O emissions were obtained in aquaculture systems with lower DO concentrations and higher feeding rates. Both nitrification and denitrification appeared to be responsible for the emissions of N2O. Key factors which correlated with the N2O emission rate in aquaculture systems were NO2-, DO and total ammonia nitrogen concentrations. (C) 2012 Elsevier Ltd. All rights reserved.