When two varieties of rice are planted together, either fully mixed, in alternate rows, or in separate halves of the same small plot surrounded by a dam, each may influence the yield of the other. The effect is as often unfavourable as favourable. However cooperation occurs, and in the case most fully investigated the yield from alternate rows was 126% of the mean yield when the varieties were grown separately. In this case at least the favourable interaction takes place mainly through water, and disappears if the area is flooded.