Technical Abstract: Switchgrass (Panicum virgatum L.), a native perennial dominant of the prairies of North America, has been targeted as a model herbaceous species for biofeedstock development. A flow-cytometric survey of a core set of 11 primarily upland polyploid switchgrass accessions indicated that there was considerable variation in genome size within each accession, particularly at the octaploid (2n=8X=72 chromosome) ploidy level. Highly variable chromosome counts in mitotic cell preparations indicated that aneuploidy was more common in octaploids (86.3%) than tetraploids (23.2%). Furthermore, the incidence of hyper- versus hypoaneuploidy is equivalent in tetraploids. This is clearly not the case in octaploids, where close to 90% of the aneuploid counts are lower than the euploid number. On a within-plant basis, almost all (93.8%) of the octaploid plants appeared to be chromosomal mosaics (having cells that vary in chromosome numbers within an individual), compared to only half of the tetraploid plants. Cytogenetic investigation using FISH revealed an unexpected degree of variation in chromosome structure underlying the apparent genomic instability of this species. These results indicate that rapid advances in the breeding of polyploid biofuel feedstocks, based on the molecular-genetic dissection of biomass characteristics and yield, will be predicated on the continual improvement of our understanding of the cytogenetics of these species.