Dates of emergence, flowering, beginning pod fill, termination of flowering, and physiological maturity were recorded for two soybean [Glycine max (L.) Merr.] cultivars from each of the maturity groups I to V. The cultivars were planted in five dates at three Missouri locations in 1971–73; at weekly intervals from April to August at Columbia, Mo., in 1972 and 1973; and at weekly intervals from May to July at Ames, Iowa and Elora, Ont. in 1973. An iterative regression analysis (IRA) technique was used to develop mathematical equations relating the rate of development to daylength and temperature. The IRA equations predicted development more accurately than calendar days or “growing degree days” at all of the locations studied.The IRA equations were also used to assess the relative effects of daylength and temperature on soybean development. Cooler temperatures and longer daylengths encountered at northern latitudes were additive in their delaying effects on flowering. The effects of cool spring temperatures on flowering predominated in the early part of the season whereas daylength predominated in plantings after June 1. Hastening effects of short days on maturity were greater than delaying effects of cool autumn temperatures. The most obvious difference among genotypes was found for sensitivity to daylength in the flowering period; later maturing cultivars were more sensitive than early cultivars.