A cytotaxonomic analysis of S. damnosum s.l. collected from three endemic sites in south-western Ethiopia and two non-endemic areas in southern and central Ethiopia, respectively, has revealed the existence of five cytoforms: ‘Kisiwani’ (a non-anthropophilic cytoform common in East Africa) and ‘Kulfo’ forms from south-western and southern Ethiopia respectively and newly recognized cytoforms ‘Bebeka’’ and ‘Jimma’ in south-western Ethiopia and ‘Sodere’ form in central Ethiopia. As ‘Bebeka’ and ‘Jimma’ are not only highly anthropophilic but also, apparently, the only anthropophilic members of the S. damnosum complex present in their respective areas, they are the main (if not the only) vectors of Onchocerca volvulus in south-western Ethiopia. While ‘Sodere’ and ‘Kulfo’ are not presumed to be vectors as they are not anthropophilic and the study sites are free from onchocerciasis. The presence of IS-1 and a complex inversion possibly involving IL-3 in all the cytoforms except in ‘Kisiwani’ strongly indicates that the cytoforms belong to the ‘Nile’ phylogenetic group of S. damnosum s.l.