In two field experiments at Deniliquin, New South Wales, time of flowering was found to be closely related to grain yield of three japonica varieties of rice ( Oryza sativa L.). Later sowing and increasing nitrogen supply delayed the mid-flowering point in all varieties. For any particular sowing time and nitrogen level Early Caloro flowered first, followed by Calrose and then Caloro II. Heavy seedling density also delayed the onset of anthesis in Caloro II at each level of nitrogen applied. Total spikelet production and total dry matter yield increased with increasing nitrogen supply. The percentage of grain filled spikelets was a major determinant of yield. Values ranged from 3 to 83 per cent and percentage decreased as flowering was delayed. There was no clear relation between temperature during flowering and low yields although critical levels were recorded.