Simple SummaryRoundworms of the genusOswaldocruziaare common parasites of the small intestine of amphibians and reptiles. Our recent study revealed that only one species,Oswaldocruzia filiformis, parasitizes anurans, lizards and snakes in European Russia. In this work we studied nematodes of the genusOswaldocruziain the European green toads from the Middle Volga region using traditional morphological and molecular genetic methods. Descriptions, original drawings, and data on molecular phylogenetic analysis for studied nematodes were produced. The results of our study showed that green toads are parasitized by twoOswaldocruziaspecies,Oswaldocruzia ukrainaeandO. filiformis. We found broad variability in morphology of the studied nematodes, which can lead to inaccurate identification of closely related species ofOswaldocruzia. The data obtained in our study should contribute to the knowledge ofOswaldocruzianematodes and host-parasite associations in general.AbstractNematodes of the genusOswaldocruziaare common parasites of the small intestine of amphibians and reptiles. Our recent molecular analysis ofOswaldocruzianematodes revealed that onlyOswaldocruzia filiformis, which possesses high morphological variability, parasitizes amphibians and reptiles in European Russia. Here we present the study ofOswaldocruzianematodes from the European green toadBufotes viridis(Anura, Bufonidae) collected at different localities of the Middle Volga region in 2018–2022. We analyzed the morphological characteristics of theOswaldocruziaspp. taxonomy together with novel molecular phylogenetic data. The data on phylogenetic analysis (based on partial CoxI mtDNA gene sequences) showed thatBufotes viridisis parasitized by twoOswaldocruziaspecies, the host-specific parasiteOswaldocruzia ukrainaeand species generalistOswaldocruzia filiformis. Broad morphological variability was revealed inO. ukrainaenematodes both from the same host specimen and from various toad individuals from different localities. Our results highlight the need for further biodiversity research of morphologically similarOswaldocruziaspecies from amphibians and reptiles in the Western Palearctic using molecular genetic methods.