Five levels of gypsum supplementation (0, 550, 1100, 2200, and 4400 kg ha-1) were applied to peanut fields 35 d after planting. After the growing season, peanuts were harvested, ground, and inoculated with 1 × 107 Aspergillus parasiticus (NRRL 5139) conidia. After 14 d at 25°C, aflatoxin was extracted and quantified by thin-layer chromatography. Fungal growth was assayed using a modified chitin assay. Peanuts from gypsum-supplemented fields at each level of supplementation supported significantly less aflatoxin production when compared to control peanuts (no calcium supplementation). Results from the chitin assay showed a decrease in fungal biomass which correlated with the decreased aflatoxin synthesis.