In a tsetse-fly dissected and examined by me in the laboratory of the Sleeping Sickness Commission at Entebbe, Uganda, it was found that not only was the intestine swarming with Trypanosoma grayi , but the proctodseum also contained vast numbers of trypanosomes. Under moderate magnification they could be seen in dense clumps attached to the wall of the proctodseum, each clump having a superficial resemblance to a patch of mould, the whole mass, however, vibrating with the movements of the flagella. It was rather uncommon to find these parasites in the hind gut, and I at once made smears from different regions of the digestive tract, and carefully preserved and stained them. As it was but a short time before my departure from Entebbe, I was not able there to do more than glance at my preparations, but I noticed at once an important fact, which since my return to England I have been able to confirm and extend by careful study of my slides: namely, that in the hind gut the trypanosomes are in process of becoming encysted. Before proceeding to describe the encystment, I will say a few words about the conditions of the occurrence of these trypanosomes, and the manner in which they were preserved. The tsetse-fly in question was one of a batch caught by our fly-boys at Entebbe on November 2, and fed the next day on a monkey infected with Trypanosoma gambiense from the cerebro-spinal fluid of a Sleeping Sickness patient. On every subsequent day these flies were fed on a healthy guinea-pig, and a certain number of flies were dissected daily and examined, until the batch was used up. Without going into details, it is sufficient to say that T. gambiense was found sparingly in the flies dissected up to 96 hours, that is to say, on November 4, 6, and 7. After this date T. gambiense disappeared completely and could not be found in any of the flies dissected. The fly in which the encystment was discovered was dissected on November 14. It must, of course, have been infected with T. grayi when caught, and, as I have stated, it had been kept 11 days in the laboratory and fed daily. At the autopsy it was found to be gorged with blood, and with all its organs perfectly normal and healthy in appearance. I examined the salivary glands and genital organs (testes and vesiculse seminales) without finding anything resembling a trypanosome. Finally the digestive tract was examined, with the results stated. No trypanosomes were found in the proventriculus.