For several years Dr F. G. Cawston of Durban, Natal, has been making examinations of various mollusks taken from the rivers and pools of the Transvaal and Natal in order to discover the fluke infection of the region. These examinations have been extensive as regards the number of individuals of each species examined, ranging up into the thousands for the more common gasteropods. The investigations have been carried on through the various seasons of the year so that the data are representative. I have been enabled to re-examine much of this material through the courtesy of Dr Cawston, who has also placed at my disposal many of the biological data concerned with these researches.