Optimum mean ambient temperatures for vegetative growth of peanut (Arachis hypogaea L.) plants are in the range of 25 to 30 C, while those for reproductive growth may be somewhat lower (20 to 25 C). Under field conditions the peanut crop is frequently subjected to temperatures in the range of 35 to 40 C, which adversely affect growth and development. Differences in heat tolerance have been found among genotypes of other crops. This was determined by the extent of electrolyte leakage from leaf discs exposed to elevated temperature treatment in vitro. These investigations were undertaken to use the in vitro leaf disc method as a means to evaluate field-grown peanut genotypes for membrane thermostability. A preliminary test in 1981 with ten genotypes showed significant differences in membrane injury among genotypes (G) and a significant day after planting (DAP) effect. However, CV's were excessive (about 38%). Modification of the procedure and method of leaf sampling reduced CV's to an acceptable level for field data (15–20%). Significant G and DAP effects were found. Hovever, G X DAP interactions were significant at P<O.05 in only one of three years of the tests, and this was due to the response of just one cultivar. Genotype differences also varied between seasons. Thus, the in vitro leaf disc method of testing for membrane thermostability appears useful for selecting peanut genotypes for improved crop tolerance to temperatures that adversely affect presently grown cultivars.