Variation in susceptibility of the snail Biomphalaria glabrata to infection by the parasite Schistosoma mansoni is, at least in part, genetically determined. Functional studies have demonstrated that hemocyte-mediated killing of the parasite involves hydrogen peroxide, the enzymatic product of superoxide dismutase (SOD). The present study identifies alleles of the gene coding for cytosolic copper/zinc SOD (SOD1). The resistance/susceptibility phenotypes and SOD1 genotypes were determined for 354 snails of the predominantly resistant 13-16-R1 strain of B. glabrata. Resistance to the parasite was found to be significantly associated with one allele of the SOD1 gene. Conversely, a separate SOD1 allele was significantly associated with susceptibility.