Recent research has indicated that avian mating systems may commonly deviate from monogamy due to extrapair fertilizations (EPFs). Because the majority of avian species have long been considered monogamous, accurate measurement of the frequency of EPFs in a variety of species is important to enhance understanding of the evolution of avian mating systems. We used DNA fingerprinting to investigate the apparently monogamous mating system of black vultures (Coragyps airaius) by assaying parentage within several nuclear families. Monogamy is suggested in black vultures because mated pairs exhibit long-term pair bonding and year-round association, and share incubation and nestling feeding duties equally. Thirtytwo breeders and 36 nestlings representing 16 complete nuclear families were tagged for individual identification and sampled for DNA analysis using 2 restriction enzymes and 3 probes for hypervariable regions. Putative parents were assigned parentage in all cases. We empirically examined the probability of detecting EPFs by comparing nestlings' fingerprints to those of a putative parent and another randomly chosen adult. All putative parents could be assigned with 95%confidence and all outside adults could be similarly excluded. There is therefore no evidence for successful EPFs in this population, indicating a mating system that does not deviate from strict monogamy. The complex social behavior of black vultures may eliminate the opportunity for EPFs due to the prohibition of copulations in the presence of relatives.