Three trematodes belonging to the genus Helicometra from the "Golfe du Lion" (Etang de Thau, France) are identified and renamed on the basis of isoenzyme electrophoresis (Reversat et al., 1989). After the bibliographical review of the genus Helicometra in the Mediterranean basin, the following results are proposed: (1) the specimens which possess the allele MDH-1a are only found in Symphodus cinereus (Labridae) : this taxon is considered to belong to Helicometra pulchella (Rudolphi, 1819) Odhner, 1902 ; (2) the trematodes bearing the allele MDH-1b parasitise S. cinereus, Gobius niger (Gobiidae), Zosterisessor ophiocephalus (Gobiidae) and Anguilla anguilla (Anguillidae) : these helminths are defined as Helicometra gobii (Stossich, 1883) Stossich, 1904 ; (3) and the digeneans defined genetically by the alleles MDH-1c and MDH-1d are found in A. anguilla, G. niger and Z. ophiocephalus, and are named Helicometra fasciata (Rudolphi, 1819) Odhner, 1902. The importance of the biochemical genetics for the study of taxonomy of trematodes is shown, but the problems these techniques raise are also discussed. The case of Helicometra gobii is typical : it exhibits a strong phenotypical polymorphism for an enzymatic type (MDH-1b). The problem of diagnostic characters for species definitions is therefore introduced.