Aflatoxins are produced as secondary metabolites under conducive climatic conditions by Aspergillus flavus. The incidence of aflatoxin varies with environmental conditions, genotype, and location. An expanded understanding of the interaction of the plant, fungus, and weather conditions is needed to further elucidate the field infection process of maize by A. flavus and subsequent aflatoxin contamination. One of the problems in evaluating maize hybrids for resistance to kernel infection and aflatoxin contamination is identifying a time period and environmental conditions that are most advantageous. Three maize genotypes (Pioneer Brand 3223, Mo18W x Mp313E, and Mp313E x Mp420) were evaluated from 1998 to 2002 in response to A. flavus inoculation and aflatoxin contamination and weather conditions favorable for aflatoxin contamination were identified. The highest aflatoxin levels were observed in 1998 and 2000 (1186 and 901 ng g(-1); P < 0.0001); while the lowest levels were detected in 1999 (39 ng g(-1)). Pioneer 3223 had significantly higher levels (1198 ng g(-1)) than Mp313E x Mp420 (205 ng g(-1)), and Mo18W xMp313E (161 ng g(-1); P < 0.0001). The hybrids had six weather-related variables in common that were positively correlated with aflatoxin accumulation. Four of these occurred during 65-85 days after planting and were temperature-related. These results suggest that regardless of the hybrid's maturity or physiological development, the time from 65 to 85 days after planting may be indicative of a period of stress which leads to greater aflatoxin accumulation at harvest.