Enzyme variation has been investigated in eight populations of Bulinus cernicus from Mauritius using isoelectric focusing and starch gel electrophoresis. Of 11 loci examined for 10 enzyme systems, seven were found to be polymorphic. These were malate dehydrogenase, glucose phosphate isomerase, acid phosphatase (A and B), 3-hydroxybutyrate dehydrogenase, 6-phosphogluconate dehydrogenase and NADH-dependent diaphorase. Invariant loci were phosphoglucomutase, xanthine oxidase, fructokinase and αaL-glycerophosphate dehydrogenase. The percentage of polymorphic loci within the populations ranged from 9.1% to 54.5%. The average heterozygosity per population for all loci ranged from 0,009 to 0.203. Genetic distance values ranged from 0.01 to 0.3 with the higher values associated with comparisons of populations from the north-west and south-east of the island. A comparison of data sets for 1980 and 1986 revealed the allelic frequencies at five enzyme loci to be remarkably consistent.