Yield under drought for several crops has been clearly established as a linear function of the cumulative amount of water transpired during the growing season. For well-watered crops, however, there are no published data on how the duration of the cropping cycle and plant population may interact to affect the relationship between yield and transpiration. We evaluated the relationship between yield and cumulative transpiration (T) or evapotranspiration (ET) for well-watered soybean (Glycine max [L.] Merr.) grown for 3 years over a wide range of maturity groups (MG, 00 through VI) and population densities (10 to 100 plants m^-2). Daily T was estimated by determining the potential ET for a given day and multiplying this by the fraction of light intercepted by the crop, and a crop coefficient. Daily values of T and ET were summed from emergence to R6. Soil evaporation estimates were made using an energy-balance approach. Yield response to cumulative T was described well by an exponential model, predicting that 90% of the asymptotic yield would be obtained at 480 mm of T. We hypothesized that the nonlinearity of yield to T was due to differences in harvest index among MG and to differences in crop vapor pressure deficit (CVPD) to which the crop was exposed during the season. Accounting for these factors, however, by regressing shoot biomass against T normalized by average CVPD also resulted in a nonlinear relationship. These data indicate that increased efficiency of water use may be realized without sacrificing grain yield through use of short-season production systems.