Patterns of fish population composition were examined in the San Pedro River, Ivory Coast, in 2001 to assess whether distinct types of assemblage exist along longitudinal gradients within this small coastal basin. A total of 45 species was found: 28 in the lower and the upper courses of the river and 29 in Faé man-made lake. The environmental variables most closely associated with assemblage-level patterns were those which can be significantly modified by dam construction: transparency, conductivity, current and sampling site position on the basin. Groups were separated by means of canonical correspondence analysis and cluster analysis. Downstream assemblages were distinguished by the presence of marine and brackish water fish species and a relatively low abundance of cichlid fishes. Assemblages from Lake Faé were characterised by a predominance of cichlids.