In the soybean cultivars'Hyuuga'and 'Fukuyutaka' inoculated with soybean mosaic virus (SMV) at the seedling stage the number of grains decreased by 40% and 24%, respectively. No decrease was observed in the plants inoculated later than one week prior to flowering. The seeds, harvested from the plants that had been inoculated with SMV at the period of flowering, showed more distinct mottling than those of the plants inoculated at the seedling stage. The rate of seed transmission of 'Hyuuga'was about 10% in the plants infected with SMV from the seedling stage to one week prior to flowering. During the week preceding flowering, the later the SMV inoculation, the lower the rate of seed transmission. No seed transmission was observed in the plants inoculated after flowering.