The objective of the present research was to study the effects of row spacing and plant population on yield and seed quality of an early maturity soybean cultivar, in late planting. Four experiments (E1, E2, E3 and E4) were carried out under field conditions, on a Ultisol (Terra Roxa Estruturada), in Botucatu, São Paulo State, Brazil. The early cultivar IAC-Foscarin-31 was planted on 12.04.86 (El), 12.22.86 (E2), 12.07.87 (E3) and 01.08.88 (E4). This cultivar was grown in interrow spacings of 0.30, 0.45 and 0.60rn and in plant populations of 200.000, 300.000 and 400.000 plants/ha in E1, of 300.000, 400.000 and 500.000 plants/ha in E2, of 400.000, 500.000 and 600.000 plants/ha in E3, and 500.000, 600.000 and 700.000 plants/ha in E4. The effects on yields, 100 seeds weight, germination and vigour of seeds (by germination first count, accelerated aging test and conductivity test) were evaluate. In El and E3, the seed yields were not affected by row spacing and plant population; in E2, the population of 500.000 plants/ha presented the highest yield in the three row spacings; in E4, only the interrow of 0.60m, with populations of 500.000 and 700.000 plants/ha, showed differences in seed production, with the lowest yields. The 100 seeds weight was generally lower in the interrow of 0.30rn than in the others. In one experiment (El), the worst physiological quality of seeds (germination and vigour) was obtained in the population of 400.000 plants!ha, mainly in 0.30m interrow spacing; in two experiments (E3 and E4), the effects of treatments on the seed quality were not evident by the tests employed: in another (E2) the seeds were demaged in their quality by high humidity at seed maturation and harvest period.