Experimental introductions of Galba truncatula in new habitats were carried out over the 30 last years in the department ofHaute-Vienne (central France) to study the details of snail settlement and the outcome of these populations, and to determine ifthe susceptibility of these new colonies to experimental infection with Fasciola hepática did not change over time. A total of 51samples (100 adult snails each) were collected from three populations known for their high susceptibility to F hepatica infections (> 60 %). Each sample was introduced in March in an open drainage furrow (36 meadows) or a road ditch (15 sites) whichhad never been inhabited by snails. Introduced snails from 22 samples survived in 9 furrows and 13 road ditches. In these sites,the number of overwintering G. truncatula increased up to the-second or third year post-introduction and progressively decreasedin the following years to become stable from the sixth or seventh year (less than 40 snails per site). In 13 populations (outof 22), experimental infections of snails with F. hepatica did not result in significant variations in survival rates or in prevalencesof infections when compared to the respective rates recorded in the three original populations. Conversely, in the nine otherpopulations, experimental infections of snails resulted in a decrease in prevalence (< 40 %), whereas the survival rates of snailsat day 30 post-exposure did not change. This last result might be explained by a change in natural parasite pressure in the newhabitats, being probably lower than that existing in the three sites inhabited by the populations of origin.

Genetic variation in the pulmonate land snail Zonitoides nitidus was examined by means of allozyme electrophoresis in 17 European populations. We observed no heterozygotes, indicating that Z. nitidus consists of several homozygous multilocus genotypes (strains) probably produced by self-fertilization. Nine strains were detected. Strains appeared not to be equally distributed over the regions considered. One strain was strongly differentiated from the others, which is suggestive of a taxonomic differentiation. Anatomically, two major phally types were distinguished: euphallics, with well-developed male reproductive organs, and hemiphallics, with weakly developed male reproductive organs. Both phally types were found together in natural populations, but euphally ratios were very low (0–19%; overall mean = 7.79%) and differed between geographical regions (Belgium, Germany, and Sweden). In some populations, hemiphallic individuals were smaller compared to euphallic individuals although the difference is small and its biological significance remains to be established. We found no clear relation between phally type and allozyme data.