Rice (paddy) and fish yields in concurrent, direct-seeded culture were investigated at the Co Do station located in the Mekong Delta, Vietnam. Three different rice seeding rates (100, 200 and 300 kg ha(-1)) were used in the absence or presence of fish (Oreochromis niloticus (all male), Puntius gonionotus, and Cyprinus carpio). Significantly (P < 0.05) higher paddy and fish yields were obtained at the lowest rice seeding rate. No significant effect of the rice seeding rate on aquatic weed biomass was found, whereas water temperature, dissolved oxygen concentration and pH were significantly (P < 0.05) higher at 100 kg rice seed ha(-1). The presence of fish resulted in a significant (P < 0.05) reduction of aquatic weed biomass, and significantly (P < 0.05) lower dissolved oxygen and PO4 concentrations, and higher chlorophyll-a levels in the pond water. Analysis of the results indicated that the effect of rice seeding rate on fish production was most likely to be related to a 'growth effect' resulting from increased availability of oxygen and food. Paddy yields at high seeding densities were affected by mutual shading. A further reduction of rice seeding rate may be feasible for rice-fish farmers, given their limited possibilities for weed control through land preparation (field levelling) and water management. (C) 1999 Elsevier Science B.V. All rights reserved.