A biosystematic study of Claytonia lanceolata and related taxa in the Rocky Mountains was undertaken to evaluate the taxonomic status of C. lanceolata var. flava. This study was part of a broader assessment to determine the need for protection of the latter taxon under the federal Endangered Species Act. Electrophoretic and morphological studies revealed that C. lanceolata var. flava in southwestern Montana and northwestern Wyoming represents a distinct diploid species (n=8) whose populations consist of yellow- and/or white-flowered plants. Morphological, allozyme, and cytological data all indicate that this taxon does not belong in the C. lanceolata complex, but is best placed in the group of narrow-leaved species that includes C. rosea, C. tuberosa, and C. virginica. Numerous populations of C. lanceolata var. flava, most often consisting of the white-flowered phenotype, were found in Montana and Wyoming, and legal protection is not warranted at this time. In some cases, actions to conserve endangered plant taxa must be preceded by an evaluation of their taxonomic status; this study illustrates the utility of biosystematic techniques in conducting such evaluations.