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A KNOWLEDGE of the genetics of resistance to leaf •*"*• rust of wheat, Triticum aestivum L., caused by the rust organism Puccinia recondita Rob. ex. Desm., would greatly reduce the empiricism involved in practical rust breeding programs and hasten production of resistant varieties. Breeding for leaf rust resistance is complicated by the occurrence of many physiologic races of the causal organism. A total of 183 physiologic races has been registered by Johnston (8). Investigations reported here were undertaken to identify by monosomic analysis the chromosomes of the eight leaf rust differential varieties that carry the gene or genes for resistance to races 9 and 15 of leaf rust. Those two physiologic races are the most common races in Kansas. REVIEW OF LITERATURE Chester (4) reviewed early studies of inheritance of leaf rust resistance in common wheat and found that, in general, leaf rust resistance was inherited in a simple Mendelian .fashion, when hybrids from pure lines of wheat were tested for reaction to pure rust races. Soliman et al.