In this study Australorbis glabratus from Puerto Rico, the Dominican Republic, Venezuela, Surinam and Brazil, Biomphalaria pfeifferi from Liberia, and B. boissyi from Egypt were exposed to strains of Schistosoma mansoni from Puerto Rico, Venezuela, Surinam and Egypt, and to seven cross-strains of S. mansoni. In many instances, incompatibilities between vectors and parasites of different endemic areas were noted. It is suggested that these incompatibilities might be accounted for in two ways: (1) interspecific and intraspecific differences between vectors in respect to the physiological factors that are responsible for the development of the parasite in the snail, and (2) physiological differences between strains of the parasite. Evidence would seem to show that there are interspecific physiological differences between Australorbis glabratus and Biomphalaria boissyi; and that there are intraspecific differences between Australorbis glabratus from Brazil and A. glabratus from other countries here dealt with. In addition, it would appear that the parasite of Egypt differs physiologically from the parasites of Puerto Rico, Venezuela and Surinam; and that the parasite of Brazil differs physiologically from the parasites of the latter countries.

We wish to express our gratitude to the following persons for their co-operation in collecting and shipping snails for this study: Drs M. Abdel Azim, C. H. Barlow, J. O. Coutinho, A. Gabaldon, the late W. A. Hoffman, G. W. Luttermoser, J. Oliver-Gonzalez, H. A. Poindexter, Bichat de Almeida Rodriques and A. E. Wolff. We are also indebted to the late Dr W. A. Hoffman, Dr M. Mayer and Dr J. Alves Meira for sending us animals infected with Schistosoma mansoni.