Collections of pupae and larvae of black flies (Diptera : Simuliidae) were carried out in 1992 and 1994 at 59 sites in four provinces of Sumatra, Indonesia. As a result, 22 simuliid species including 12 species newly recorded from Sumatra were identified, bringing the total number of simuliid species recorded from Sumatra to 26. All these species were assigned to the genus Simulium s. 1. and placed into three subgenera, i.e., Gomphostilbia (12 spp.), Nevermannia (3 spp.), and Simulium s. str. (11 spp.). Further groupings were made at the species-group level within each subgenus. The simuliid fauna of Sumatra is Oriental in character, being very closely related to those of Peninsular Malaysia and Java. Infections with mermithids, microsporidians and/or fungi due to Coelomycidium sp. were found in the larvae of 10 of the 22 black-fly species examined.