1. The only biting fly of common occurrence in Mauritius is Stomoxys nigra.2. The method of transmission of surra by Stomoxys nigra is direct, i.e., it occurs when flies take a meal on a surra-infected animal, with trypanosomes in the peripheral blood, and immediately afterwards feed on a healthy animal.3. Trypanosomes are found active in the proboscis of Stomoxys from one to ten minutes after a meal, when the temperature of the room is between 25 and 27° C.4. No active form of trypanosomes can be found in the gut, Malphigian tubes or salivary glands of the fly 24 hours or longer after a meal on an infected animal.5. For successful transmission the interval between the feeding on the infected and on the healthy animal must not be more than ten minutes.6. Trypanosomes in the gut of Stomoxys are active from one minute to one hour after a meal on a surra-infected animal.7. Trypanosomes are partially disintegrated, the cytoplasm becoming more granular and the flagellum disappearing, 24 hours after an infected meal.8. No developmental forms of trypanosomes have been found in the salivary glands, gut or Malphigian tubes of Stomoxys nigra 24 hours or more after an infected meal.9. When trypanosomes could not be found in the peripheral blood of a surrainfected animal, transmission could not be effected.