Frogeye leaf spot (FLS) of soybean [Glycine max (L.) Men.] is caused by the fungus Cercospora sojina Hara. The fungus is ubiquitous, but only problematic in hot humid soybean-producing regions such as Brazil, China, Nigeria, and the southern USA. Significant yield losses (10-50%) are commonly associated with FLS epidemics. The quantification of unique alleles for resistance within the southern germplasm pool is an essential step toward developing a more usable set of differential genotypes and thereby clarifying the race situation within the C. so/ina-soybean interaction. Our objective was to determine the inheritance of resistance to FLS in PI54610 and Peking and their allelic relationship to Res,. 'Lee' soybean was used as a susceptible parent for crosses and control in all experiments. Parents and F 2 seedlings were inoculated with a C. sojina spore suspension in the greenhouse or field and then rated for disease development 14 to 21A later. On the basis of segregation ratios (3:1 resistant/susceptible in Peking X Lee and PI54610 x Lee, and 15:1 in 'Davis' X Peking and PI54610 X Peking), we found resistance in Peking was determined by a single dominant gene nonallelic to Res,. We also found, based on nonsegregation of resistance within the Davis X PI54610 population, that PI54610 has the same gene as in Davis (Res,). Resistance in Peking should be considered unique for the purpose of race differentiation and as a commercial source of resistance to FLS should Res, fail. R OGEYE LEAF SPOT OF SOYBEAN is caused by the phyto-, 'athogenic fungus C. sojina Hara (Hara, 1915). Cercospora sojina infects leaves, stems, and seeds of soybean (Sinclair and Backman, 1989). The pathogen has a worldwide distribution, with significant yield losses reported in