The massive localization of metacercariae of the trematode Opisthioglvphe ranae (Frolich, 1791) in the tail of R. arvalis larvae and the presence of tail anomalies suggest that this parasite can initiate external morphological deviations of amphibians. Five variants of anomalies were identified in R. arvalis larvae. The residential zone of the urban agglomeration is characterized by the greatest variety of variants of anomalies, which is probably due not only to the characteristics of the development of amphibians, but also to the specifics of the anthropogenic environment: high mineralization, eutrophication of water bodies, changes in pH, temperature conditions. The most common option is tail shortening, the proportion of which is higher in the forest-park zone (80%).