All available soybean genotypes of Maturity Group III (515 lines) from the USDA soybean germplasm collection were evaluated for degree of daylength sensitivity at Urbana, Illinois, during the summer of 1973. Each genotype (planted May 19) was grown in four-plant hills, spaced 24 x 40 inches, under both natural daylength and natural daylength extended by 5 hrs incandescent lighting. The incandescent lights were placed 5 feet above ground surface and provided illumination during the night period in the intensity range of 5 to 10 ft-c (50 to 100 lux).