Because of the problems encountered in the classification of snails of medical importance, biochemical methods have been sought to help clarify the situation. Of these, the separation of the enzymes of adult snails by electrophoresis seems the most promising but very few attempts have been made so far to use the results for taxonomic studies. The purpose of the present study was to evaluate the use of the enzyme systems of neotropical planorbid and of lymnaeid snails to elucidate their taxonomy and also snail-schistosome relationships at the species and population levels.The findings show the characteristic electrophoretic patterns of digestive gland esterases of the planorbid and lymnaeid snails used, as well as their variation and the level of such variation among certain populations and the consistency of the patterns among others. The results also show that, in general, the extent of variation between some populations of the same species is greater than the differences between species of the same group. However, at the specific level, there are similarities suggesting close relationships between some populations of Biomphalaria glabrata and B. tenagophila on the one hand, and of certain populations of B. peregrina and of B. obstructa on the other hand.The present study has thrown some light on the question of electrophoretic variation in enzymes, and the ways in which this can be applied to studies of the genetics of snails. A correlation is suggested between certain patterns that indicate biochemical similarities or differences among the planorbid snail populations and the susceptibility of the species or the population to infection with the schistosomes.