We study the phylogenetic relationships among some North American Colias ("sulfur") butterflies, using mitochondrial gene sequences (ribosomal RNA, cytochrome oxidase I+II) totaling about 20% of the mitochondrial genome. We find that (1) the lowland species complex shows a branching order different from earlier views; (2) several montane and northern taxa may be more distinct than in earlier views; (3) one morphologically conservative Holarctic assemblage, C. hecla, is differentiated at the molecular-genetic level into at least three taxa which occupy distinct positions in the phylogeny and are sisters to diverse other taxa. These conclusions, constituting phylogenetic hypotheses, are supported by parsimony, maximum-likelihood, and Bayesian reconstruction algorithms. They are tested formally, by interior branch tests and paired-site tests, against alternative hypotheses derived from conventional species and subspecies naming combinations. In all cases our hypotheses are supported by these tests and the conventional alternatives are rejected. The "barcoding" subset of cytochrome oxidase I sequence identifies only some of the taxa supported by our full data set. Comparison of genetic divergence values among Colias taxa with those among related Pierid butterflies suggests that species radiations within Colias are comparatively younger. This emerging Colias phylogeny facilitates comparisons of genetic polymorphism and other adaptive mechanisms among taxa, thereby connecting micro- and macro-evolutionary processes.