Six corn samples from each of Iowa's 99 counties were collected in December, 1983, to determine the degree of aflatoxin contamination in 1983 Iowa corn. While the statewide average was 18.8 parts per billion (ppb), 23% of the counties had no detectable aflatoxin and 11% were over 100 ppb. Aflatoxin concentrations were highest in the southeast and southcentral regions, coinciding with the extreme drought conditions suffered there during August. The coefficient of variation among samples within counties averaging over 20 ppb was 105%. The data support other reports that high temperatures and drought stress in the three to four weeks following silking promote aflatoxin contamination. The accuracy of a whole- kernel black-light test as a presumptive test for aflatoxin concentration in excess of 20 ppb was examined for two sample sizes (10.9 and 1.8 kg). The black light identified all 10.9-kg samples that contained aflatoxin contamination above 20 ppb. For the 1.8-kg samples, 11% of samples with aflatoxin levels above 20 ppb were missed. Of the large samples with bright greenish-yellow fluorescent particles, 55% contained less than 20 ppb compared with 33% of the small samples.