Prevalence, intensity and abundance of protostrongylid larvae in snails and slugs were studied on 10 sheep and goat pastures in the region of Stara Zagora, Southern Bulgaria, in 2001. Eight gastropod species were infected: Helicella obvia, Cernuella virgata, Monacha cartusiana, Zebrina detrita, Cepaea vindobonensis, Chondrula microtraga, Helix pomatia and Deroceras reticulatum. Four nematode species were identified: Muellerius capillaris, Neostrongylus linearis, Cystocaulus ocreatus and Protostrongylus sp. Helicella obvia was the most common and abundant snail, and showed the highest infection parameters. Cernuella virgata was the only species in one pasture and had high infection descriptors. Monacha cartusiana and Zebrina detrita had high infection levels in some pastures. A subset of data representing the infection of H. obvia was used to test for relationships with environmental characteristics. Protostrongylid infections in this host species showed considerable variation between sites. M. capillaris had the highest prevalence and intensity, followed by C. ocreatus and Protostrongylus sp. Comparisons of sites at lower vs at upper elevations did not show significant differences in infection, with the exception of Protostrongylus sp. The primary vegetation was the only explanatory variable for the total prevalence of protostrongylids. This variable, in a combination with altitude, mollusc species richness and sample size explained 73% of the variation of mean intensity. In the models explaining variations in the prevalence and the mean intensity of the nematode species, 9 variables were included: primary vegetation, altitude, annual precipitation, mean annual precipitation, mollusc and nematode species richness, concentration of inorganic nitrogen in soil, snail population density and ratio between the number of goats and the number of sheep grazing on pasture. H. obvia was considered a suitable screening species for assessing the pasture contamination in the region.