The Tugela-Vaal River water scheme constitutes a possible source for the introduction of bilharziasis into an area where the disease is at present not endemic. A survey was made of the snail species in the area encompassed by the scheme. Although Bulinus (Physopsis) africanus (a host of Schistosoma species responsible for urinary bilharziasis) was found in the proposed source area of the water scheme, it is not possible at this stage to determine with certainty whether this host will be introduced over the Drakensberg escarpment into the Orange Free State. However, this is a distinct possibility, and it is discussed in relation to ecological factors.