Lake Sonfon lies in an isolated basin in the Sula Mountains of Sierra Leone. One hundred and two animal taxa were collected from this lake in April 1976: 78 of these have been identified to species, and all but four as far as the genus. The fauna consists mainly of animals with extensive geographical distributions, indicating that they have wide dispersal capacities. Three of the rotifers have not previously been recorded in Africa, and are known only from the Americas. The lake is characterized by a low conductivity and low pH. These two factors influence the fauna. Snails are absent, rhizopods are abundant, and so are rotifers of the genusLecane, while the genusBrachionusis poorly represented. When compared with Debundsha Lake, a low conductivity crater lake in Cameroon, which has been studied to a similarextent, Lake Sonfon has twice as many species; this can be related to the greater diversity of habitats in Lake Sonfon.