Fourteen field trials were conducted on farmers' fields in five villages of the Union Territory of Delhi, India during 1995–96 and 1996–97 to study the effect of introducing a crop of summer mungbean on the productivity of a rice–wheat cropping system. Timely sown summer mungbean yielded 0·4–1·3 t/ha protein-rich grain and, on average, increased rice yields by 0·5–0·9 t/ha and the yields of the succeeding wheat by 0·4–0·7 t/ha. The yield response of rice to the summer mungbean crop was greater when less urea-N fertilizer was applied, and N applied to the rice did not decrease the residual effect of the summer mungbean on the succeeding wheat. The increase in total productivity of rice–wheat cropping system due to summer mungbean ranged from 0·3 to 2·6 t/ha with an average of 1·6 t/ha.