Abstract: To predict effects of environment and genotype on soybean (Glycine max (L.) Merr.) yields under field conditions, a full understanding of photoperiod effects on growth and development throughout the soybean life cycle, especially after flowering, is needed. Two field experiments were conducted by using different planting dates to investigate effects of photoperiod on soybean growth during and after flowering for the1992 and 1993 growing seasons. Growth stages of 10 soybean strains differing in maturity dates that included determinate and indeterminate genotypes were recorded throughout growing seasons. Results indicated that photoperiod affected all stages of soybean growth and development and in some genotypes, this included pod set and seed filling. Longer photoperiods induced more vegetative growth and delayed the maturing process. Later plantings (after mid- June) resulted in significantly fewer flowers, pods and seeds per plant and in significantly lower yields. The mechanism of photoperiod effects on soybean growth before and after flowering was similar in a quantitative way. This effect was realized through the alteration of the photosynthate partitioning processes between vegetative and reproductive growth. Due to accelerated reproductive processes under short photoperiods and high temperatures, the accumulation of dry matter slowed down or even reversed during the late reproductive stages for early maturing strains or other strains planted late. The vegetative status of and late MGs (IV or later) at R1 stage was strongly correlated with final yields + in both determinate and indeterminate strains. and Information gathered from this study will prove valuable in building a better simulation model for soybean production.