I investigated variation among 46 populations of the Fox Sparrow (Passerella iliaca) using five microsatellite loci, to determine whether populations assorted into the four groups (red, slate-colored, sooty, and thick-billed) previously defined on the basis of morphology and mitochondrial DNA (mtDNA). Results of neighbor-joining and principal components analyses showed that the red, sooty, and thick-billed phenotypic-mtDNA groups were mostly differentiated from each other. Some populations of the slate-colored group were intermingled with other groups. Thus, there was not complete agreement between microsatellites and the mtDNA gene tree. Some of the populations of slate-colored Fox Sparrows that aligned with other groups were geographically adjacent. A plot of principal components 1 and 2 showed populations of the slate-colored group surrounded by members of the other three groups, which mirrored their geographical arrangement. The lack of clear differentiation in microsatellites could be due to insufficient time for nuclear allele sorting or hybridization at group boundaries; it is likely that both are explanations. The four groups qualify as phylogenetic species that exhibit hybridization, whereas the number of biological species, although greater than currently recognized (one), depends on the amount of introgression deemed acceptable.