Recent studies have indicated that cottonseed (Gossypium hirsutum L.) germination and seedling vigor are more closely related to seed density than to any other physical properties such as size or weight. This study was undertaken to determine the physical and chemical differences among seed of different densities within a cultivar and among seed of the same density between cultivars. The primary objective was to relate physical or chemical properties to seed density and to offer an explanation for the reported correlation between seed density, germination, and seedling vigor. Cottonseed of 10 cultivars grown on the Texas High Plains were separated into 4 density groups using a pneumatic separator and increasing air velocities. Separation of seed by density was somewhat independent of cultivar; whereas, seed weight was highly dependent on the cultivar. As seed density increased, the proportion of the total seed weight attributable to the embryo increased from 50% to 65% across all cultivars. The lipid concentration increased significantly from the lowest to the second density group, then remained essentially constant thereafter. All chemical constituents increased significantly when expressed on the basis of seed content. A significant cultivar effect existed within each density group with respect to the quantity of material present. Seed density was a good indication of seed maturity, as shown by the ratios of embryo weight to total seed weight. Although the concentrations of the various chemical constituents were only slightly affected, the quantity of both organic and inorganic material available to the germinating seedling was increased as seed density increased.