A. pringlei (Scribn. et Smith) Hitchc., a high-elevation perennial range grass of California (USA), behaved cytologically as an allotetraploid (2n = 28). Similar seed set in the absence and in the presence of foreign pollen demonstrated that A. pringlei is normally self-fertilizing. A. pringlei hybridized readily with tetraploids (2n = 28) A. scribneri Vasey, A. dasystachyum (Hook.) Scribn., and A. violaceum (Hornem.) Lange; it hybridized with difficulty with diploid (2n = 14) A. spicatum (Pursh) Scribn. et Smith. Chromosome pairing at metaphase I in the A. pringlei .times. A. spicatum hybrid, 8.69I, 5.98II, and 0.12III, indicated that A. pringlei contains a genome partially homologous with the A. spicatum genome. Chromosome pairing in hybrids of A. pringlei with A. scribneri, A. violaceum and A. dasystachyum showed that both A. pringlei genomes are more or less homologous with the genomes of the 3 tetraploid species. Chromosome pairing was most complete in the A. pringlei .times. A. dasystachyum hybrids, which averaged 1.23I, 13.31II, 0.03III and 0.01IV in 179 metaphase I cells. The basic genome formula of A. pringlei and the other tetraploids may be written as SSHH, but the genomes of each species are unique with respect to structural chromosome rearrangements. All hybrids except A. pringlei .times. A. dasystachyum were totally sterile. Natural introgression between A. pringlei and the other four species is highly unlikely because of hybrid sterility and geographic separation. Distinct morphological differences and almost absolute reproductive isolation from its closest relatives warrant the treatment of A. pringlei as a separate species.