Adult female Simulium damnosum s.l. flies obtained in nine countries of the Onchocerciasis Control Programme were examined for 14 morphological characters with the aim of selecting specific characters to identify the main species that form the complex. Two approaches were used, the first involved discriminant function analysis of the species groups using all the characters which were both qualitative (the colour of the fore coxa, the antennae, the wing tuft, the arculus, the ninth abdominal tergite setae and the scutellar setae) and quantitative (the lengths of thorax, antenna, and femur, tibia and first basitarsus of both the fore and hind legs) in nature. The second sought colour characters unique to the S. damnosum subcomplex and to S. yahense, to separate them from the rest, and progressively applied discriminant analyses in the dichotomous stepwise manner to within the subcomplex level. The combination of pale arculus and pale antennae with pale or dark fore coxae were unique to the S. damnosum subcomplex, whilst those of black arculus together with wholly dark or mixed ninth abdominal tergite setae and scutellar setae were unique to S. yahense. Further discrimination was achieved between subcomplexes and species by entering the qualitative and quantitative characters into discriminant function analysis. The two schemes were both successful at separating the main species subcomplexes of the S. damnosum complex, with 90-100% correct classification. Within the subcomplex level, correct identification of sibling species was 85-100% successful.(ABSTRACT TRUNCATED AT 250 WORDS)