Blood meal volume was determined for two species of mosquitoes, Aedes albopictus (Skuse) and Culex tritaeniorhynchus Giles, recently demonstrated to be natural vectors of canine heartworm in Japan. When fed on unrestrained dogs, volumes were from 1 nl to 6 microliters in both species, but percentages of engorged mosquitoes were 45.2-64.3% for Ae. albopictus and 11.9-22.2% for Cx. tritaeniorhynchus. Ae. albopictus preferred to feed on human bait versus canine bait when both were exposed concurrently.