Trisetum spicatum (Poaceae) is circumboreal as well as bipolar in distribution. The species is comprised of both tetraploid and hexaploid cytotypes that display high morphological diversity. Previous taxonomic treatments have resulted in the separation of the species into a series of species, subspecies, varieties, races, and types. Many of these treatments were inconclusive, based on highly variable characters, and few specimens were examined. For instance, the characters of the inflorescence and relative stem pubescence are the characters most relied on for subspecific separation. These characters have been found to be extremely inconsistent, even within the same population.

This study is concerned with the North American component of the species complex. Three-hundred and fifty herbarium specimens and field collections were measured for thirty-three characters. These data were analyzed numerically with basic statistics, cluster analysis, principal coordinate, and discriminant function techniques. The results indicate that extreme variation exists within the species complex with no distinct subgroups. The species was also analyzed genetically with isozyme electrophoresis. Eight populations were surveyed for five enzyme systems. These results indicate the species to possess complete allelic homogeneity within populations with few interpopulation differences. Previous recognition of the hexaploid as a distinct species is not supported morphologically or genetically.