Two cotton modules were constructed near Odem, Texas, and two near Yuma, Arizona from producer fields in areas with histories of aflatoxin problems. Also, two modules were constructed in Mississippi from a field artificially infested with Aspergillus flavus. Aflatoxin levels increased significantly with time in one module from Texas and one from Mississippi. The relationship between aflatoxin level and time was positive but not significant for all other sites except TX-2. The significant decrease of aflatoxin level over time in TX-2 was attributed to termperatures unfavorable for aflatoxin production.