M.Sc. / The Lesotho Highlands Water Project (LHWP) is bound to adversely affect the ecology of the Orange River system and in particular the highland tributaries. The Senqunyane River (a tributary of the Orange River) would be no exception. Water extraction by means of the proposed Mohale Dam (Phase 1B of LHWP) is expected to alter the downstream habitat of the Senqunyane River, which is alarming as the rare rock catfish (Austroglanis sclateri) occur in these waters. Another concern is the lack of knowledge on the riverine ecology as no research has been conducted in the Senqunyane River downstream from the proposed Mohale Dam wall. In order to gain baseline information on the aquatic habitat, distribution and composition of the fish community as well as habitat parameters of the rock catfish , four surveys (from Spring 1994 to Summer 1996) were conducted in the Senqunyane River, major tributaries and in the Senqu River. This information is needed for future monitoring of downstream alterations to the riverine ecosystem and to determine the habitat preference of A. sclateri. Water quality variables were generally in accordance with the guidelines of Kempster, Hattingh & Van Vliet (1980) and the Department of Water Affairs and Forestry (1993), but excessive levels of un-ionized ammonia and phosphates were recorded at antropogenically disturbed areas. Two distinctly different fish communities were respectively found upstream and downstream from the Semongkoaneng Waterfall as this waterfall serves as a migration barrier. The upstream community consists entirely of Psemlobarbus quathlantbae (Maloti minnow), while the downstream community consists of Barb us amens (Smallmouth yellowfish). Labe() capensis (Orange River mudfish). Austraglanis .cclateri (rock catfish) and Oncorhunchus An ecological investigation of the aquatic vertebrates below the proposed Mohale Darn Wall in the Senqunyane River, Lesotho, with emphasis on the rock catfish (Austroglanis sclateri) al1111111Hry mykiss (Rainbow trout). B. aeneus has the highest-representation within this community while 0. mykiss has the lowest representation. Low numbers of the latter species is probably related to the temperature regime and bottom siltation and therefore the Senqunyane River can be regarded as of a marginal area for the occurrence of trout. A. sclateri also has a low representation within the community which is probably a result of its specific habitat requirements as apposed to 13. aeneus and L. capensis which utilize a wider variety of habitat types. Distinctly different habitat preferences were observed for different A. sclateri lifestages. Recruits preferred backwater pools, juveniles preferred suckles and adults preferred runs. It is clear that juveniles and adults prefer strong current velocities and would probably not do well in conditions of reduced flow. A. sclateri could therefore be regarded as an indicator species in the context of instream flow requirements for the Senqunyane River below the proposed Mohale Dam wall.