Species exist as biological entities with patterns of discontinuous phenotypic variation. However, the distinctness of taxa is called into question when morphological intermediates exist in areas of sympatry, reflecting either gene flow among variants of a species or hybridization between different species. Studying the partitioning of genetic variation provides a means to discern between the two possibilities. We used genetic and morphometric approaches to investigate the degree of isolation among the three members of the Lycaena xanthoides species group. Lycaena xanthoides, L. editha, and L. dione are predominantly allopatric and have been treated both as three separate species and as a single polytypic species. Using 618 bp of the mitochondrial gene COII, we found little phylogenetic resolution, but significant among-taxa genetic variance partitioning. Divergence among these taxa has been relatively recent, as evidenced by relatively low pairwise sequence divergence. Also, the existence of two well-supported clades within L. xanthoides sensu stricto, concordant with the Transverse Ranges of southern California, indicates divergence within this taxon, and a possible cryptic species. Significant morphological differentiation between L. editha and L. xanthoides supports the hypothesis that these taxa represent separate gene pools. Populations occurring in a narrow zone where the two species’ ranges approach are characterized by intermediate morphology, suggesting incomplete morphological divergence or recent hybridization. These findings highlight the utility of genetic data in inferring species boundaries and the identification of cryptic lineages.