Bullnus globosus is the only known vector of Schistosoma haematoblum in Zimbabwe. The population genetic structure of this vector snail from the two main river drainage systems, represented by 27 localities, was determined from starch gel lsoenzyme electrophoretic data. Out of 11 enzyme systems screened eight loci were resolved, and six of them were polymorphic in at least one of the populations sampled. Genotypic distributions showed deviations from Hardy-Weinberg proportions due to deficiency of heterozygotes and this was attributed to selfing. The Save River drainage system, which falls under the high rainfall area, revealed a higher proportion of gene diversity than the Sanyatl River drainage system. Hierarchical analysis showed that differentiation within the populations in the two drainages (FSR = 0.085) was higher than between the two drainages (FRT= 0.011). This highlights the role of ecological factors (drought/desiccation and floods), genetic drift, and seasonal gene flow as the main factors influencing the genetic structure of these populations.