In the report of the recent conference in Kampala, Uganda, “Zoonoses in East Africa”1, I was quoted as suggesting that Trypanosoma rhodesiense had developed from Tr. brucei by repeated passages through Glossina pallidipes. My thesis was, in fact, almost the exact opposite of this. I emphasized that all the evidence so far shows that the property of infectivity to man which alone distinguishes Tr. rhodesiense from Tr. brucei is an extremely stable one and that there has never been a known case of interconversion of these two. On the other hand, I pointed out, in any area where Tr. rhodesiense had appeared, either it could be traced to direct introduction by infected human beings from a known source, or the other human infective ‘species’, Tr. gambiense, had been known to be present in the area for some time.