This study was carried out to investigate the yield response of 2 soybean (Glycine max (L.) Merrill) cultivars to different planting dates and plant populations and to examine the relationships of seed yield with leaf area index (LAI) and total dry matter (TDM) during the vegetative and reproductive periods for planting dates. plant populations, and cultivars. Two soybean cultivars, A-3127 (early maturity group, MG-III) and 1530 (late maturity group, MG-IV). were planted in 2 planting dates; mid-April and mid-May during 2005 and 2006 at high (660,000 plant ha(-1)) and low (330,000 plant ha(-1)) plant populations. Plantings in mid-April increased seed yield by about 10.0%-13.5% compared with mid-May plantings. High plant populations produced 17.7% higher seed yield than the low population. Seed yield of the late maturity cultivar was more than that of the early maturing cultivar. Significant and positive relationships between seed yield and LAI were evident for planting dates at all growth stages in 2005 and 2006. Seed yields at high and low plant populations were negatively and significantly correlated with LAI at vegetative stage (V(5)), but positively and closely related to LAI at reproductive stages (R(4) and R(6)) in both experimental years. The relationships between seed yield and LAI with early and late maturity cultivars were significant and positive at all growth stages except the V. growth stage in both 2005 and 2006. Seed yield was positively and significantly correlated with TDM at all growth stages for planting dates, plant populations. and cultivars.