Results are reported of enzyme analyses, by isoelectric focusing in polyacrylamide gels, of adult Schistosoma haematobium worms derived from 22 isolates originating from 13 countries. Polymorphisms have been identified in the glucose-6-phosphate dehydrogenase (G6PD) and phosphoglucomutase (PGM) systems. Certain forms appear to be restricted in their geographical distribution and their occurrence outside their usual areas suggests human population movements resulting in mixing of parasite strains. The possible implications of minor variations in some PGM patterns and the apparent absence of heteropolymer fractions in presumed G6PD heterozygotes are discussed. The use of the technique to detect natural multiple miracidial infections in snails is reported and discussed.