Smooth bromegrass (Bromus inermis Leyss.) has both tetraploid (2n = 28) and octaploid (2n = 56) ploidy levels that have been difficult to characterize cytogenetically because of similar chromosome morphology. Objectives of this study were to identify individual chromosomes of tetraploid and octaploid B. inermis with C-banding procedures along with chromosome length and arm length ratios, develop more detailed karyotypes than those previously available, and use the karyotypes to examine the genomic relationship of tetraploid and octaploid B. inermis. Root tips of the plants from four tetraploid and three octaploid accessions were used to produce chromosome squash preparations for cytogenetic analysis. The tetraploid B. inermis genome consisted of 12 chromosomes with a telomeric band on each arm and sixteen chromosomes with only one telomeric band on one arm. All of the chromosomes of the tetraploid form, except for four chromosomes, were identified by C-banding patterns, chromosome length, and arm length ratio. The octaploid B. inermis genome consisted of four chromosomes with no C-bands, ≈14 chromosomes with two telomeric bands, and ≈38 chromosomes with only one telomeric band on either the short or long arm. The combined use of C-banding, chromosome size, and arm length ratio only enabled groups of 2, 4, 6, or 8 similar chromosomes to be identified because of similarities in chromosome morphology of the octaploids. Results indicate that tetraploid B. inermis is an allotetraploid since all chromosomes except four could be separated into identifiable pairs. Because of differences between expected and actual numbers of satellite chromosomes and chromosomes with specific C-banding patterns, octaploid B. inermis is probably not a doubled form of the tetraploid B. inermis
