Technical Abstract: Peanuts become contaminated with aflatoxin when subjected to prolonged periods of heat and drought stress. The resulting aflatoxin contamination costs the peanut industry over $20 million annually. The development of peanut cultivars with resistance to preharvest aflatoxin contamination (PAC) would reduce these costs. Two requirements are needed to breed a cultivar with resistance to PAC. First we must have screening techniques that can reliably differentiate genetic resistance from susceptibility. During the course of this project we have developed field screening techniques that can measure genetic differences in aflatoxin contamination. The second requirement is genetic variation for resistance. During the course of this project we have identified 11 core accessions that have shown at least a 70 % reduction in PAC in multiple environments. We have also identified significant reduction in PAC in peanut genotypes with drought tolerance. These sources of resistance to PAC have been entered into a hybridization program. They have been crossed with cultivars and breeding lines that have high yield, acceptable grade, and resistance to tomato spotted wilt virus (TSWV). Due to the large environmental variation in PAC, it is not feasible to examine these breeding populations until late generations when there is less heterozygosity and adequate seed are available for field testing using multiple replications. We have identified families and individual breeding lines that have relatively low PAC, relatively high yield, and acceptable levels of resistance to TSWV. However, much faster breeding progress could be achieved through the development and use of indirect selection techniques. We are exploring this with studies on mechanisms of resistance to PAC and attempting to develop molecular markers for resistance.