Observations were made in Sierra Leone from September 1949 to November 1951 on the incidence of trypanosomes in Glossina palpalis (R.-D.), in continuation of those previously reported that began in September 1948. Flies were dissected; trypanosomes found to be restricted to the proboscis were classed as Trypanosoma vivax and those found also or only in the mid-gut as T. congolense. None of the flies had infected labial glands. The whole data are presented for the sexes separately by months.

The existence of a marked seasonal variation in the incidence of T. vivax in G. palpalis is confirmed. The highest rates of infection occured in the months of heavy rain, and there was a significant inverse correlation between the monthly means of infection rate and of hours of sunshine. The cause is unknown, but possible explanations are discussed.

The older the fly, as judged by its appearance, the greater the rate of infection by both species of trypanosme; the virtual absence of infection in the youngest group of flies does not support the theory that they can only be infected when very young.

In the case of T. vivax, but not in that of T. congolense, females flies were more heavily infected than males. This may be due to larger food intake by the females, or to males, because of their more active life, appearing to be older than they are.