The taxonomic status of the 2 morphologically distinct forms of Dall's porpoises, Phocoenoides dalli (dalli- and truei-type) remains unresolved. To address this uncertainty, mitochondrial DNA (mtDNA) sequences were used to estimate phylogenetic relationships between the 2 types and, in conjunction with microsatellite markers, to test genetic differentiation between truei- and dalli-type populations. Twenty-three truei-type and 113 dalli-type specimens were sequenced for 379 base pairs of the mtDNA control region and genotyped for 6 microsatellite loci. Twelve haplotypes were shared between truei- and dalli-types. A neighbor-joining tree of mtDNA haplotypes showed 2 distinctive clades, each containing individuals from both types. This suggests that truei- and dalli-types are forms of the same species. However, at the population level, statistically significant genetic differentiation was found between truei-type and sympatric dalli-type populations. These results argue that differentiation between truei- and dalli-types is at the population level, much in the same way that dalli-type populations differ among each other.