Historically, a large number of taxonomic forms has been recognized within Nearctic pikas (Lagomorpha: Ochotonidae; Ochotona), including up to 13 species and 37 subspecies. After 1965, 2 species and 37 forms have been recognized: the monotypic O. collaris of Alaska, British Columbia, Yukon, and Northwest Territories, and O. princeps, with 36 subspecies spread throughout western Canada and the western United States. The 36 subspecies of O. princeps have been distinguished by subtle differences, particularly in pelage coloration and body size, within the highly fragmented distribution of the species on isolated "islands" of cool, rocky habitat. However, molecular phylogenetic studies (allozyme electrophoresis and sequencing of both mitochondrial and nuclear genomes) indicate the existence of 5 phylogenetic lineages within O. princeps. The cohesiveness of each lineage has been reinforced during glacial stages by introgressive hybridization among currently isolated populations within each lineage. In contrast, the low level of cranial variation and lack of consistent differentiation in cranial characters, pelage coloration, or body size among the 5 lineages indicates idiosyncratic interlocality variation due to high inbreeding within highly isolated populations, genetic drift, and possibly selection for common traits. Examination of allozymic, morphological, and nuclear DNA data indicates previous introgressive hybridization among several of the lineages, probably associated with contact during the Last Glacial Maximum. Herein we characterize morphometric variation between and among O. collaris (n = 164) and O. princeps (n = 1,999) and revise the subspecific taxonomy of O. princeps to 5 subspecies based on molecular phylogenetic lineages, at least 3 of which are known to possess a unique dialect in the short call: O. p. princeps (Northern Rocky Mountains), O. p. fenisex (Coast Mountains and Cascade Range), O. p. saxatilis (Southern Rocky Mountains), O. p. schisticeps (Sierra Nevada and Great Basin), and O. p. uinta (Uinta Mountains and Wasatch Range of central Utah). These 5 subspecies represent evolutionarily meaningful units for consideration of possible management applications if populations of O. princeps are imperiled by human activities. [ABSTRACT FROM AUTHOR]