Twelve locations of Mytilus galloprov~ncialiws ere sampled in lagoons and along the seaside in southern France. Lagoons represent a habitat with widely varying ecoloqcal conhtions. Each lagoon has its own characteristic range of sahities and temperatures which fluctuate throughout the year. We studied several fundamental parameters of the population genetics of M. galloprov~cialisa t 8 enzymatic loci (genetic differentiation between locations, heterozygote deficits, selection, linkage disequilibrium) and investigated whether they are influenced by the high spatial and temporal instability of the lagoons (brackish water). In contrast to what could be expected based on the corresponding literature, we found no evidence for genetic differentiation, either among lagoons or between lagoons and the seaside. There was, thus, no evidence of selection on any of the loci studied.