Samples of airborne grain dust representing a variety of grains were examined for the presence of aflatoxins. Although aflatoxins could not be detected in most of the samples, one corn sample contained 130 ppb aflatoxin B1. When the components of this sample were separated according to aerodynamic diameter and analyzed for aflatoxins, it was found that particles with diameters 7–11 urn and <7 μm had much higher levels of aflatoxins than the sample as a whole. These findings suggest that measurements of aflatoxin in whole corn might underestimate aflatoxin levels in corn dust aerosols.