A polymerase chain reaction (PCR) heteroduplex assay (HDA) was developed to identify avian derived mosquito blood meals to the species level. The assay used primers amplifying a fragment of the cytochrome B gene from vertebrate but not invertebrate species. In Culex tarsalis fed on quail, PCR products derived from the quail cytochrome B gene were detected seven days post-engorgement. In an analysis of wild-caught mosquitoes, 85% of blood-fed mosquitoes produced detectable PCR products. Heteroduplex patterns obtained from bird-derived PCR products were found to permit the unambiguous identification of all species examined. No intraspecific variation in HDA patterns was found. The PCR-HDA was used to characterize blood meals in wild caught Cx. tarsalis. Of the 67 blood meals analyzed, 60% were derived from avian sources. Of the avian blood meals, 65% were derived from a single host, the common grackle.