The rate of escape of the third-stage larvae of Brugia pahangi from Aedes togoi which were allowed to probe on a cat and a mouse at time intervals of 5, 10, 20, 30 and 60 seconds was studied. The rate of escape of third-stage larvae at all time intervals was greater during probing on the cat than on the mouse, and was in a logarithmic linear relation to the length of probing time by the mosquito host. The greatest loss (91.35%) of third-stage larvae was in mosquitoes which fed on a cat until fully gorged. There was a remarkable rate of escape (57.41%) within 5 seconds. This striking rate of escape of third-stage larvae may have important implications on filariasis transmission. Most larvae migrated towards the proboscis and head of mosquitoes during these feeding periods. Nearly equal numbers of third-stage larvae escaped from mosquitoes which fed to repletion for more than 60 seconds on a mouse and from those which probed fro more than 60 seconds on the same mouse but did not engorge. This confirmed our view that filling up of the stomach with blood does not constitute the single factor in causing the release of third-stage larvae from the mosquito host.