Received for publication December 19, 2003. Aflatoxin, produced by the fungus Aspergillus flavus Link, reduces the value of corn (Zea mays L.) and is usually associated with high temperatures, water stress, and insect damage. The objective of this study was to determine if Bt (Bacillus thuringiensis) corn hybrids or 'tropical' germplasm could reduce aflatoxin accumulation with later planting dates. Aflatoxin accumulation (B1, B2, G1, and G2) in corn grain was evaluated on Bt, non-Bt, and tropical non-Bt hybrids and four planting dates (March, April, May, and June) from 1998 to 2000. Aflatoxin concentration in corn varied across years but generally decreased with later planting date. In 1998, aflatoxin accumulation was lower in Bt (314 ng g?1) than non-Bt hybrids (634 ng g?1) but not different than tropical non-Bt hybrid (470 ng g?1). However, aflatoxin contamination was lower from Bt hybrids (70 ng g?1) than from the tropical non-Bt hybrid (259 ng g?1) but not different in non-Bt hybrids (86 ng g?1) in 1999. In 2000, aflatoxin levels were low, and hybrid had no effect on aflatoxin concentration. Temperature and irrigation effects on aflatoxin accumulation were not consistent. Increased temperature and delayed harvest may lead to aflatoxin accumulation before harvest. However, precipitation may influence aflatoxin levels in some years. The results of this study indicate that aflatoxin accumulation in corn may be decreased with later planting date, and less accumulation in Bt than non-Bt or tropical non-Bt hybrids may be indirectly explained by insect reductions. 