Citation: Abbas, H.K., Shier, W.T., Cartwright, R.D. 2004. Effect of heat stress on aflatoxin and fumonisin production in corn (maize, zea mays) in arkansas. Phytopathology. 94:S1 Technical Abstract: A severe infestation by aflatoxin-producing fungi diminished the food quality of the southern US corn (maize) crop in 1998. Commercial corn hybrids planted at the same and other locations in Arkansas (21 in 1998; 29 in 1999; 15 in 2001; some planted multiple years) were evaluated for resistance to mycotoxin contamination from natural infection by Fusarium spp. and Aspergillus spp. At harvest, kernel corn samples were evaluated for the presence of aflatoxins and fumonisins. In 1998, samples from all hybrids exceeded 20 ppb aflatoxin (range: 21-699 ppb) and 2 ppm fumonisins (23-79 ppm), the maximum levels permitted for some uses by United States Food and Drug Administration guidelines. In 1999 aflatoxin levels ranged from none detected in most hybrids to 255.3 ppb, and fumonisin levels from 0.3-8.1 ppm. In 2001 the fumonisin levels were very high in all hybrids (range: 8-83.6 ppm), whereas aflatoxin levels were low (<5 in most hybrids, ranging up to 131 ppb). The presence of aflatoxin (AFB1 and AFB2) in samples was confirmed by TLC and LC/APCI/MS and fumonisins (FB1, FB2, FB3, FB4 and FC4) by LC/ESI/MS. Arkansas experienced unusually high day and nighttime temperatures in 1998, but nearly normal temperatures in 1999 and 2001. The results are consistent with heat stress having an important effect on mycotoxin production in corn by A. flavus and Fusarium spp.