The influence of harvest date on aflatoxin contamination of Deltapine 90 cottonseed was studied in the Yuma Valley, Arizona in 1987. Aflatoxin levels in naturally contaminated cottonseed collected from the lower third of the canopy increased from 96 ng/9 to 763 ng/g between mid-September and late-October. Increases occurred after boll maturation and primarily within locks which did not fully fluff out. Bolls inoculated with A. flavus via simulated pink bollworm exit holes contained high aflatoxin levels (24,000 ng/g) but, these levels did not increase between harvest dates. Extensive initial contamination of susceptible tissues in inoculated bolls may explain differences between inoculated and naturally infected bolls. Laboratory and greenhouse experiments were used to define environmental conditions that may lead to increased contamination of cottonseed. The results suggest effective management of aflatoxin contamination of cottonseed is dependent upon timely harvest.