Seeds of seven hybrid and open pollinated sorghum cultivars were separated with sucrose density solutions into four specific gravity (SG) classes for studies on the effects of seed SG on field performance. The seeds were planted at different densities to provide three levels of competition: "normal" planting density; one high and one low SG seeds in the same hill; and single seeds in widely spaced hills, no competition. There were no differences among the high, medium-low and unseparated (control) seed SG treatments for emergence, vegetative and reproductive development for the normal density plantings. Yield, however, was significantly lower for plants of the unseparated (control) seeds as compared to the SG separated seeds. When high and low SG seeds were planted in the same hill, emergence of the high SG seeds was higher and they produced more vigorous plants that produced more tillers, matured earlier and yielded more as compared to those from low SG seeds. In the case of high and low SG seeds planted at non-competitive spacing, emergence, plant height, number of tillers and yield were higher and the number of days to panicle exsertion and anthesis were fewer for the plants from the high SG seeds as compared to those from the low SG seeds. Priming of high SG seeds did not enhance plant performance above the level of the high SG unprimed seeds. Seed SG was greatly affected by the degreee of weathering and immaturity. As the degree of weathering or immaturity increased, the percentages of seeds in the two highest SG classes sharply declined, while those for the two lowest SG classes sharply increased. Seeds from the intense weathering treatments exhibited cavitation and profuse fungal growth in the placental sac area and in the embryo. Similar cavitation was observed in seeds from the immaturity treatments. Differences in the speed of germination among the unweathered, mature, high SG seeds appeared to be related to differences in permeability of the pericarp to water and/or oxygen.