The 18S rDNA sequences of the six most common European Lymnaeidae species (Mollusca:Gastropoda:Basommatophora) have been obtained by direct PCR cycle sequencing and silver staining methods. The sequence alignment and secondary structures of the 18S rRNA gene of Lymnaea stagnalis, L. auricularia, L. peregra, L. palustris, L. glabra, and L. truncatula are analyzed. This gene proves to be a good marker for both specific determination and supraspecific lymnaeid phylogeny. The malacological importance is evident, considering the specific determination problems of individual snails and the present systematic chaos in Lymnaeidae due to their pronounced morphoanatomic uniformity, which makes a classification by traditional methods impossible. The majority (17) of the total of 43 nucleotide-substituted positions appears to be confined to a small region included in helix E10-1 of the variable region V2, enabling species group distinction: (1) the first sequence is common to L. auricularia and L. peregra; (2) the second sequence is unique to L. truncatula; and (3) the third sequence is identical for L. glabra, L. palustris, and L. stagnalis. The other 26 nucleotide-substituted positions are dispersed over the entire gene, although four grouped nucleotide positions in helix 6 of V1 are of interest in distinguishing L. glabra from both L. palustris and L. stagnalis. The phylogenetic trees obtained by comparison with four other molluscan species (a polyplacophoran, two bivalves, and a stylommatophoran gastropod) show the presence of four well-defined subgenera among the genus Lymnaea sensu lato: (1) Lymnaea (Radix), (2) Lymnaea (Galba), (3) Lymnaea (Leptolimnaea), and (4) Lymnaea (Lymnaea). Two branches, L. auricularia-L. peregra-L. truncatula and L. glabra-L. palustris-L. stagnalis, are worth mentioning from the parasitological point of view, since the two digenean species of large medical and veterinary impact transmitted by lymnaeids, Fasciola hepatica and F. gigantica, appear to be linked to the first branch.