A survey of freshwater snails on Mafia Island has been conducted that mapped the distribution and, by using a timed searching strategy, documented the relative levels of abundance of species. Twenty-five freshwater habitats around the island were surveyed; snails were found in 15 sites (60%). From the total number of snails collected (n=240), prosobranchs (59%) outnumbered pulmonates (41%). Potential intermediate hosts of urinary schistosomiasis were encountered and these Bulinus species were further characterized by analysis of six enzyme systems (GPI, PGM, MDH, HBDH, GDH & AcP), by inspection of DNA sequence variation within mitochondrial cytochrome oxidase sub-unit I (COI) and nuclear ribosomal internal transcribed spacer (ITS). Three species of Bulinus were found: a single B. africanus group taxon, B. nasutus, and two B. forskalii group taxa, B. forskalii and a species presently identified as Bulinus sp. in this paper. The genera Pila, Lanistes and Bulinus were widespread across the island and dominated the fauna. The remaining taxa were found in lower numbers and often only at single sites. Three prosobranchs: Lanistes purpureus, Pila ovata and Melanoides tuberculata were found. In addition to Bulinus, the pulmonates included a small discoidal planorbid (?)Ceratophallus and an ancylid (a freshwater limpet). The factors that shape the dispersal and distribution of molluscs on Mafia were considered. Water chemistry could be an important variable as both pH (mean 6.37±0.99 SD) and conductivity (mean μS=149±135 SD) were significantly different from similar habitats on neighbouring Zanzibar Island (P<0.001). The Mafian fauna is clearly associated with that of mainland Africa and Madagascar.