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Chromosome counts were made from meiotic material of B. gracilis plants from 108 localities in western Texas and eastern New Mexico and one locality in western Oklahoma. The previously unrecorded number 2n = 20 was found in plants from thirty-six collections, all but one of which were from the area between the Pecos River and the Rio Grande. Plants from fifty-six localities over a wide area of the upland plains of eastern New Mexico and western Texas, the Guadalupe and Davis Mountains of trans-Pecos Texas, and the mountains of central New Mexico were found to have the tetraploid number 2n = 40. Maximum chromosome association in the tetraploids varied from almost complete bivalent pairing to almost complete quadrivalent association. The hexaploid number 2n = 60 was found in plants from eleven localities in New Mexico on the high upland plains west of the Pecos River and in the Abo Mountains. The hexaploids exhibited a maximum of five hexavalents plus a variable number of quadrivalent associations. The aneuploid number 2n = 42, presumably derived through non-disjunction in the tetraploids or through aneuploid variation at the diploid level and subsequent polyploidy, was found in plants from six localities. The single collection from Oklahoma yielded plants which had 2n = 84 and regularly exhibited 42 bivalents. It has been assumed that the basic chromosome number in the genus Bouteloua is x = 7, but the present material indicates that a basic number of x = 10 is more likely for the genus. It was not possible to demonstrate any consistent correlation between growth habit or other morphological characters and chromosome number, probably because B. gracilis is a polyploid complex with several diploid members and marked by varying degrees of auto- and allopolyploidy. Ecological and geographical factors exhibit some correlation with chromosome number. The polyploids have a wider geographical range than do the diploids, but the ranges of the two overlap widely. The polyploids also appear to have a wider range in relatively more recently available habitats. The diploids, on the other hand, exhibit a wider range of ecological adaptation than do the polyploids.