The vector efficiency of the Haiti and Covington strains of Culex quinquefasciatus after feeding on dog blood infected with Dirofilaria immitis (approximately 110 microfilariae (mf)/20 microliters of blood) was 1.2 and 0.3%, respectively, versus nearly 20% for the Vero Beach strain of Aedes taeniorhynchus. At a much higher microfilaremia (approximately 400 mf/20 microliters), it was 1.6, 0.5 and 31.5%, respectively. The poor vector efficiency of the 2 Cx. quinquefasciatus strains probably was due to the formation of long, needle-like oxyhemoglobin crystals in the blood meal, which prevented the migration of microfilariae to the Malpighian tubules. Crystals did not form when Cx. quinquefasciatus ingested D. immitis microfilariae mixed in human blood.