Indigobirds (Vidua spp.) are host-specific brood parasites that have diversified in a recent radiation apparently driven by host colonization. Behavioral imprinting of both male and female indigobirds on host song is thought to promote rapid speciation because it results in assortative mating between indigobirds associated with a particular host. We conducted a song playback experiment to test whether male indigobirds discriminate among potential competitors based on song. Of particular interest was the behavior of two sympatric host races of the Cameroon indigobird Vidua camerunensis that differ only in host song mimicry and other components of their vocal repertoires. Territorial males of the two V. camerunensis host races and Vidua raricola, a morphologically distinct indigobird species, were tested with playbacks of each other's songs. Males of all three groups responded most aggressively to songs of their own species and/or host race, as evidenced by strong and highly significant differences in a variety of response variables. This differential territorial defense suggests that an intruding male with different songs does not represent a competitive threat and is expected if females mate assortatively with respect to song. Thus, our results provide evidence of premating reproductive isolation among recently evolved indigobird species and host races.