The microfilaria density of periodic Wuchereria bancrofti was compared in venous blood, finger-prick blood and blood ingested by Culex fatigans. Blood samples were taken by venipuncture and finger-prick over a 24-hour period, and fully engorged female mosquitoes were collected from each of two carriers for 15 minutes before and after each blood sample was taken during the night. There was no difference between the microfilaria density in venous and finger-prick blood during the study period. Moreover, rates of increase and decrease in microfilaremia were the same in the two types of blood. The data suggest that there was no obstruction of the microfilariae by the capillary beds and therefore no backlog of microfilariae on the arteriole side. Mosquitoes feeding on the carrier with a high microfilaremia ingested, on the average, fewer microfilariae than expected whereas those females feeding on the carrier with a lower density ingested, on the average, about the expected numbers of microfilariae.