-Eight species of of the genus Rana, comprising five species endemic to western North America and three related species, were examined electrophoretically in order to investigate their systematic interrelationships. Two morphotypic stream frogs, R. boylii and R. muscosa, were found to be sister taxa derived from within an assemblage of brown frogs that included R. aurora, R. cascadae and R. pretiosa. Rana cascadae was closely allied to R. aurora. The two subspecies of R. aurora were discernable, but closely related, and exhibited variation conforming to a north-south pattern of allozyme divergence. Allozyme differences indicated considerable divergence in R. pretiosa, both between samples and from other species. The five western species were found to be a monophyletic group exclusive of the Eurasian species R. temporaria and R. dybowskii and the North American R. sylvatica. The 24-chromosome species R. dybowskii was very distant genetically from all other species examined. The electrophoretic evidence agrees with a number of conclusions drawn from karyological investigations, such as the common derivation of R. boylii and R. muscosa, but disputes previous conflicting systematic hypotheses concerning these based on immunological distance studies and reanalyses of existing data. The widespread brown frog morphotype may be considered a conserved primitive state in Rana. In certain respects, karyotypic, morphotypic and genotypic evolution in these may have proceeded independently. [Anura; Ranidae; Rana; systematics; evolutionary relationships; allozymes; electrophoresis; divergence; western North America.] The five species of endemic to far western North America can be resolved, in general terms, into two morphotypic groups: brown frogs (Rana aurora, R. cascadae and R. pretiosa) with dark facial masks and prominent dorsolateral folds, and stream frogs (R. boylii and R. muscosa) without these features (Zweifel, 1955; Chantell, 1970; see also Green, 1986). Despite various studies, the relationships of these to one another and to other species groups of Rana have not been adequately resolved. Case (1978a), using both isozyme electrophoresis and albumin immunology, investigated the systematics of the five western species of Rana (Figs. 1A, B, 2A). Case's electrophoretic study indicated a clear distinction of the western Rana from Mexican and eastern North American Rana. This prompted her to redefine Zweifel's (1955) R. boylei (sic) species group (that had included such Mexican species as R. tarahumarae and R. sinaloae [=R. pustulosa]) to comprise the five western species exclusively. This alignment had previously been suggested by osteological (Chantell, 1970) and immunological (Wallace et al., 1973) data. However, Case's (1978a) electrophoretic analysis (Fig. 1A) seemed to indicate that R. aurora was more closely allied to R. boylii and R. muscosa than to the other brown frogs (R. cascadae and R. pretiosa). This conflicted with what would have been the expected relationships predicted from comparative morphology (see Green, 1986). Case (1978a) considered her immunological data inconclusive concerning the five western species and presented them as an unresolved pentachotomy (Fig. 2A). A reappraisal of Case's immunological distance matrix was offered by Farris et al. (1979). Their tree of relationships, promoted on the basis of its lesser percent standard deviation (% SD; Fitch and Margoliash, 1967), contradicted Case's (1978a) isozyme analysis, Case's immunological analysis, and previous morphological conclusions of Zweifel (1955) and others. Furthermore, Farris et al.'s (1979) assessment seemed to indicate paraphyly of the group by including the European species R. tem-