A cytogeographio survey of western AmericanRumex paucifoliusNutt. reveals that diploid (2n= 14) and tetraploid (2n= 28) populations occur within the Rocky Mountain var.paucifoliusand also within the California var.gracilescensRech. The meiotic behavior of the chromosomes, the occurrence of aneuploid individuals in the populations, and the occasional occurrence of higher polyploids in nature suggest that the tetraploids are substantially autopolyploid in origin. The two diploid populations are restricted in range and are known, respectively, from high elevations in the southern Sierra Nevada of California and the mountainous region at the headwaters of the Missouri River in Montana and Wyoming. They are patroendemics, for they represent the ancestral diploids from which the more abundant polyploids arose. Both the diploid and the tetraploid populations are dioecious. The available evidence supports the suggestion that sex determination follows an X/Y (Melandrium) scheme in inheritance, but no evidence of heteromorphic sex chromosomes was found.Rumex paucifoliusis cytologically and morphologically distinct from any other native North AmericanRumex. Its characters resemble theR. acetosellagroup of the subgenus Acetosella, as much as they resemble any known members of subgenus Acetosa. For this and other reasons, the subgenera Acetosa and Acetosella should be combined.