Simuliid flies are a well known feature of riverain land in the northern Sudan during months of low Nile. They have been discussed in detail by Lewis (1948). Two species occur, Simulium damnosum Theobald, and S. griseicolle Becker. The former is larger and darker in colour. The latter is smaller with characteristic grey shoulders and is usually much more numerous. S. damnosum bites humans readily, especially round the ankles. S. griseicolle is usually less effective in its attempts to bite humans but susceptible persons may be badly bitten. It commonly attacks the head and shoulders and may cause great irritation by invading the eyes, ears, nose and mouth. Temporary protection for up to four hours can be obtained by rubbing the skin with di-methyl phthalate which acts as a repellent against both species.