Ways for better management of rice-fish ecosystems can be brought about from an understanding of ecological interactions of rice and fish. One approach to study this is through ecological modeling of nitrogen (N) flows in the ecosystem. Nitrogen is described as the kingpin in rice farming. Its flows in lowland irrigated ricefields could possibly be influenced by growing fish in ricefields. Bioperturbation of the soil by fish may increase the thickness of the aerobic zone and make nutrients more available for plant use. Fish as a component of the rice ecosystem could make a difference in improving the sustainability of rice production than when rice is cultured alone. Thus, steady-state N models for rice and rice-fish systems in lowland irrigated ricefields were constructed using ECOPATH II in view of the major role of this nutrient in increasing rice yield. The N flows impacts and interactions of the different ecosystems components particularly rice and fish were differentiated and assessed. The analysis of the models revealed that: (1) higher N transfer efficiencies were obtained in all trophic levels, suggesting that fish helps in improving N utilization within the ecosystem; (2) the rice-fish system has a higher capacity to produce and capture N within the system than rice monoculture; (3) rice showed a negative impact on natural fish food; (4) Nile tilapia and common carp did not improve rice yield; and (5) common carp showed better performance in rice pest control than Nile tilapia. The fish, however, negatively affected oligochaetes and zooplankton in the ecosystem.