Streams and rivers on the Vanuatu islands of Espiritu Santo, Pentecost, Éfate and Tanna were sampled for gastropods and the physical and chemical characteristics of the water were investigated between 25/11/95 and 13/12/95. Twenty three species of neritid and forteen of thiarid prosobranch gastropods were found, together with the pulmonate Physastra nasuta and an opisthobranch species Strubellia. Six thiarid, two Clithon and one Neritina species, which are present in the Solomon Islands and South East Asia, but which are absent further south on New Caledonia and the Fiji islands, were found on the Vanuatu islands. On the other hand, Septaria bougainvillei and Clithon pritchardi, which are present on the Fiji islands and New Caledonia, were found no further north than the southern Vanuatu island of Tanna. This suggests that Vanuatu is a transition zone for both northern and southern freshwater snails. Éfate had many more species (26) than the other islands (16-19-. This was partly explained by its central geographical position and the higher ion content of its streams.