Biomass production in well watered soybean has generally been considered to be a linear function of cumulative light interception. Recent studies have shown, however, that there is a curvilinear response of biomass accumulated during a growing season as a function of cumulative intercepted radiation. This also suggests that radiation use efficiency may decrease as the total amount of light intercepted increases. This study specifically evaluated radiation use efficiency throughout a growing season of different maturing soybean cultivars at several sowing dates.