Transgenic Bt cotton may have reduced susceptibility to aflatoxin contamination as a result of pink bollworm resistance. During 1995 and 1996, Bt cottonseed from several commercial fields in Arizona contained aflatoxin levels unacceptable for dairy use. Comparison of cottonseed with and without BGYF (bright-green-yellow fluorescence) from one highly contaminated (>6,000 ppb aflatoxin B₁) Bt seed lot indicated that most contamination probably resulted from exposure of mature cotton to high humidity. Seed exhibiting BGYF was repeatedly detected in Bt cottonseed lots but, pink bollworm exit holes were not observed in the field. A field plot test in 1996 demonstrated high resistance among Bt cultivars to both pink bollworm damage and formation of BGYF seed cotton. These observations suggest that resistance to pink bollworm will result in reduced aflatoxin contamination when pink bollworm pressure coincides with conditions conducive to Aspergillus flavus infection. However, Bt cultivars are not resistant to aflatoxin increases occurring after boll opening and large quantities aflatoxin can form during this period.