The flight performance of three cytospecies from the West African complex of Simulium damnosum Theo. was tested under controlled laboratory conditions. A total of 293 individuals (174 of S. squamosum (End.), 65 of a new form of the subgroup of S. soubrense Vajime & Dunbar and S. sanctipauli Vajime & Dunbar and 54 of S. sirbanum Vajime & Dunbar) was tested on 526 occasions, of which 380 involved sucrose-fed insects, 65 water-only-fed insects, of both sexes, from laboratory-emerged stock and 81 blood-fed, wild-caught females. A high percentage of tests resulted in flights. Frequency distributions of both flight duration and distance flown were skewed, more flights being of short than of long duration. Sucrose-fed females and females caught wild at human bait generally flew well. Insects given access to water only made short flights, but S. sirbanum flew fairly well under these conditions. The recorded maximum flight durations and distances were 262 min (in a male) and 5·25 km (in a female) in S. squamosum, 282 min (in a male) and 6·15 km (in a male) in S. soubrense/sanctipauli, and 186 min (in a female) and 5 27 km (in a female) in S. sirbanum, all from sucrose-fed groups. Some insects were flown on several days up to 11 days after emergence. Some insects made long flights on one day but not on others. Further long flights could be induced on the same day if the insect was allowed access to sucrose solution prior to retesting. These results are discussed in relation to previous findings from S. ornatum Mg. s.l. and to the migration potential of the important vectors of onchocerciasis in West Africa.