Summary and Conclusions 1. Experiments with Taeniorhynchus (Mansonioides) africanus Theo. showed that this species is capable of acting in the capacity of insect host for the yellow fever virus. Transmission to monkeys was accomplished in ten instances with eight fatalities, both by the bites of mosquitoes and by the injection of their ground-up bodies. Five of the deaths resulted from the inoculation of ground-up insects, and three from infection by bites. A recovery also occurred after each method of exposure. One insect was sufficient to cause typical fatal infection. 2. Similar experiments with Anopheles gambiae Giles (= costalis Lw.) failed to produce infection in test animals through bites or by injection of the ground-up insects after a period equivalent to that required by A. aegypti to become infective by bite. 3. To test the persistence of the virus in A. gambiae, injections of their ground-up bodies were made on alternate days after they had fed on infected animals. Fatalities occurred, after lengthened incubation periods in the test monkeys, after the two and four-day injections in one experiment in which units of 5 insects were used each time. In the other experiment, a severe immunizing fever developed following the inoculation of 10 insects twenty-four hours after their initial blood-meal. Injections after longer intervals were negative, all animals later proving susceptible. 4. Attempts to determine the incubation period of the virus in T. africanus failed because of an apparently low percentage of infection among the insects, the shortest positive transmission by biting occurring at sixteen days. Parallel tests with control losts of A. aegypti in small numbers proved positive in eight days twice and in nine days once from the time of the original feeding. 5. It appears that A. gambiae is not concerned in yellow fever transmission in West Africa. On the other hand, T. africanus must be definitely considered in the development of a control campaign for this disease, as it is an important domestic mosquito in the adult stage, biting humans readily, and is capable of producing an experimental infection similar to that produced in M. rhesus by the normal insect host, A. aegypti.