Information on the effect of planting date on soybean composition in the Early Soybean Production System in the mid-south U.S.A. is limited. Therefore, a field experiment was conducted to evaluate the effect of planting date and cultivar differences on protein, oil, and fatty acids in selected soybean [(Glycine max (L) Merr.] genotypes.DK 3964 and AG3905 of maturity group (MG) III, Pharoah, Egyptian, and DT97-4290 of MG IV, and DT99-16864 of MG V were used. In 2005 planting dates were 23 April (early), 7 May (intermediate), and 21 May (late); in 2006 planting dates were 18 April(early), May 15(intermediate), and May 25(late).The results across years and across cultivars showed that protein was significantly higher in the late planting than early planting. However, oil was significantly higher in the early planting than the late planting. Unsaturated (oleic, linoleic, and linolenic) fatty acids were more stable than saturated (stearic and palmitic) fatty acids. Individual cultivars responded differently to planting date for seed composition. For example, protein, oil, and fatty acids composition in DT99-1686, Egyptian, and Pharoah showed stability across planting dates. Whereas, total oil in AG3905 and DK3964 was higher in early planting than late planting. Protein in DT97-4290 was higher in late planting than early planting. Fatty acids were relatively stable across years. The results indicate that planting date affected seed composition, and that the stability of seed composition across planting date depends on the cultivar. Therefore, planting date and cultivar selection are important depending on whether high protein or high oil is desired.