Gastropode molluscs L. saxatilis and L. obtusata were investigated in two sites of the White Sea coast (Ryazhkov Island and Levin Navolok, Kandalaksha Bay). The molluscs in the studied sites were heavily infected with trematodes parasites (about 65 % for L. saxatilis and 50 % for L. obtusata). The frequency of the shell shape abnormalities in both species were 7-8 % in the studied sites. The shell spires of these shells were extremely high with «balloned» whorls probably due to downward displacement of the shell sutures. All snails with abnormal shell were infected by microphallides of «pygmaeus» group (mainly M. piriformes) at the final stage of development. The parasites prevalence of the molluscs with the normal shell was significantly lower. The morphological analysis of shells revealed that the spire height in infected snails increased as compared to uninfected ones. All these data confirm that the shell shape deviations of littorines were caused by the trematodes. However, many of the littorines infected by M. piriformes hadn't got any shell deviations. These cases seemed to de connected with the recent infestation. Here the shell increment was not yet affected by the parasites. This suggestion was supported by the investigation of the shell shape in the molluscs contained the early development stages of the The morphometrical analysis also showed that the shell shape of periwinkles was dependent on the type of habitat (intertidal zone and substrate type). The relative spire height was the highest in L. obtusata and L. saxatilis from the upper zone (L. saxatilis especially from the stones). Thus, specific microhabitat may affect on the molluscs shell independently or intensify the influence of trematode infection.