1. An experiment in bush-clearing was carried out at a small endemic focus of onchocerciasis in South Kavirondo district with a view to observing its effect on the adult population of S. neavei.
2. The clearing consisted of the complete removal of undergrowth and partial removal of trees, from the banks of fly-infested portions of two confluent rivers, the Riana and the Yabe.
3. In the first year of the experiment the clearing was carried out on the infested section of the Riana River, after which there was a decrease in the incidence of S. neavei on this section compared with that on the uncleared Yabe River.
4. In the second year a similar clearing was effected on the infested section of the Yabe River, after which the S. neavei incidence on both sections came together again and during the third year showed a steady increase.