Seed size is a function of grain fill rate and duration. This study was conducted to determine the genetic control of these traits in sorghum [Sorghum bicolor (L.) Moench]. Twenty random recombinant inbred lines, the original parents and two unrelated lines were crossed to four females using a Design II mating design. Individual heads were tagged at 50% flowering and rate of grain fill was determined by harvesting heads during the linear phase of grain fill and regressing grain weight on accumulated heat units. Grain fill duration was estimated visually and by dividing final grain weight by grain fill rate. Estimates of genetic variances within this population showed additive genetic variance to be largely responsible for both traits. Estimates of combining ability effects showed large differences among the lines for additive genetic effects with little dominance effects. Improvement of both traits will be possible and will be necessary in both male and female parents if seed size in hybrids is to be increased.