The remarkable variation in plumage and morphological characters in the redpoll complex Carduelis flammea-hornemanni has puzzled taxonomists for several decades. In contrast, molecular studies have not revealed any genetic differentiation among the phenotypic redpoll forms. This could either be a result of high present-day gene flow or morphological differentiation following a rapid and recent population expansion. We sequenced a major portion (960 bp) of the mitochondrial control region in individuals of the two taxa Carduelis flammea flammea and C. f. cabaret. Birds were sampled on autumn migration in southern Sweden (n = 30) and on breeding areas in southern Norway (n = 11). We found 22 variable sites defining 26 different haplotypes, of which most (22/26) were singletons. The level of haplotype and nucleotide diversity was low in the two taxa and we found no evidence of genetic differentiation. A mismatch distribution was very similar to that expected from a sudden population expansion model. Our estimates suggest that the redpoll population expanded during the last glaciation episode from a small population to a long-term effective population size of 230 000 females. The findings in our study suggest that the morphological differentiation between the two taxa occurred rather recently but after the population expansion.