Females of Culex pipiens L. experimentally infected with Wuchereria bancrofti were tested for in vitro transmission of infective stage L3 larvae using a capillary tube technique. Of the females harbouring L3 larvae, 60.7% (34/56) transmitted a geometric mean of 2.84 L3 larvae (range 1-35). Transmitting Cx. pipiens contained significantly more L3 larvae than non-transmitters. Both the probability of transmission and the number of L3 larvae transmitted were related to the total number of loads larvae/mosquito. Almost all females with L3 loads > or = 10 transmitted larvae in vitro. The possibility is discussed that the detection of L3 larvae in field collected mosquitoes by dissection may overestimate the proportion capable of parasite transmission and the proportion of L3 larvae deposited on host skin by Ca. 40%.