SUMMARY.1. Five populations of the cladoceranSimocephalus vetulus(O. F. Muller) examined by enzyme electrophoresis at four to five loci revealed high levels of genetic variation.2. The one intermittent population had genotype frequencies in Hardy Weinberg equilibrium, the three permanent populations showed deviations from equilibrium of varying magnitude. The fifth population was fixed for a single clone, and was possibly recently derived from a single ephippial hatchling.3. Laboratory breeding experiments confirmed that this species is a cyclic parthenogen.