Previous studies suggest grain sorghum, Sorghum bicolor (L.) Moench, acts as an early-season predator source for nearby cotton, Gossypium hirsutum L., in areas where their growing seasons overlap. However, few data exist on predator movement in this system, and proposed causes of movement have not been tested. Field studies in 2001 and 2002 addressed both issues. Predator marking with rubidium was employed to measure predator movement between equal-sized areas of cotton and grain sorghum at three stages of grain sorghum phenology. Concurrent manipulative experiments in field cages tested for effects of phenology and aphid levels on movement by Hippodamia convergens Guérin-Méneville, a common predator in this system. Results from 2001 showed cotton gained 2.7 predators for every one lost to adjacent grain sorghum but that the collective movement of predators was similar among the three periods examined. The coccinellids H. convergens and Scymnus loweii Mulsant moved preferentially into cotton and seemed responsible for the overall pattern of predator movement between crops. For predators moving from grain sorghum into cotton, estimated rates of dispersal (15.8–19.9 m/d) were found to be similar among all taxa studied. Cage experiments suggested both crop phenology and abundance of aphid prey in cotton and grain sorghum cause predator movement, but only the effect of phenology was consistent between years. These results support the idea that grain sorghum is a source of predators during cotton’s early growth stages and suggest that grain sorghum may continue to contribute to natural enemy populations during later stages of cotton growth.