Variation in phenological, morphological and physiological traits related to seed yield in soybeans may be the result of environmental and management factors. Therefore, information available from previous planting date and cultivar studies in Iowa, and information collected from a field experiment established in 1983, were used in this study;According to simple correlation analyses, environmental factors related to crop phenology and yield better than the management factors. Planting date was the only management factor strongly related to crop phenology and yield, but its influence was only indirect, as planting date determines the crop environment to a great extent;The duration of all the phenological events studied, except the length of the vegetative period, was strongly related to heat units. This may indicate that photoperiod effects are stronger in the preflowering phase. The length of the vegetative phase and the length of the period from beginning bloom to beginning seed were reduced as a result of drought stress. On the other hand, the length of the seed filling period was increased as a result of drought stress. Further research under strictly controlled drought levels is required, however;Relative leaf growth rate was positively correlated to relative growth rate. Both variables were inversely related to heat units and solar radiation. Leaf growth rate was positively correlated with total plant weight at beginning seed and negatively correlated with planting date;Results from the field experiment and path coefficient analysis seemed to support a causal scheme where the total plant weight at beginning seed determines seed number directly and duration of the effective seed filling period indirectly. A multiple regression equation having seed number and effective filling period as predictor variables for seed yield gave an R('2) = 0.86. The amount of total variation in seed yield explained by each of these variables was 66 and 20%, respectively;Covariation in seed size and seed number explained yield differences between "new" and "old" cultivars.