Selected soybean [Glycine max (L) Merr.] lines of maturity groups (MG) III, IV, and V showed percentage differences in total oil, fatty acids, and total protein in response to planting dates. Although planting early maturity (MG, III, IV and V) soybean in midsouthern U.S. results in maximum yield, little is known about the effect of planting date on seed composition. The objective of this study was to assess if early and late planting dates affect the stability and content of oil, fatty acids and proteins. The six lines used were: AG3905 and DK3964 (MG III); DT99-16864, Egyptian, and Pharoah (MG IV); DT97-4290 (MG V). The lines were planted on 23 April 2004 (early planting), 7 May (intermediate planting), and 21 May (late planting). Planting date had a significant effect on stability and distribution of individual fatty acids in AG3905, DK3964, DT99-16864, and Egyptian. For example, in DT99-16864 the percentage of oleic acid to total oil increased from 20.55% at early planting date to 25.38% at late planting. Similar pattern was observed for palmitic in DT97-4290 ( 3.55 versus 4.19%) and AG3905 (4.14 versus 4.85%), and for linolenic in Egyptian (6.66 versus 7.80%). However, linoleic in DT99-16864 decreased from 56.6% at early planting to 52.6% at later planting. Total proteins in Egyptian increased significantly from 38.8% at early planting date to 41.2% at late planting date, while its oil decreased from 20.7% at early planting to 18.3% at late planting. This proportion shift in protein-oil was significant, as indicated by the protein:oil ratio. Planting date did not change significantly the total oil, fatty acids, and total protein in DK3964 and Pharoah. Further research is being conducted in 2005 to confirm the results.