A series of laboratory experiments was carried out to determine the susceptibility of Biomphalaria alexandrina alexandrina, the snail vector of schistosomiasis in the United Arab Republic and the Sudan, to a strain of Schistosoma mansoni from Mwanza, Tanzania. The objective of the investigation was to determine whether or not B. alexandrina is refractory or partially refractory to infection with strains of S. mansoni other than the Egyptian strain.In one series of 5 exposures, snails from a colony originating in Alexandria, UAR, showed a mean infection rate 4 times that observed when snails from the same colony were exposed to the local strain of S. mansoni.