The first finding of Apophallus müehlingi (Jägerskiöld, 1899) Lühe, 1909 (Trematoda, Heterophyidae) in Karelia.
Data on the finding of Apophallus müehlingi (Jägerskiöld, 1899) Lühe, 1909 in gulls of Lake Ladoga and its morphological characteristics are provided for the first time. In common gull (Larus canus), this parasite occurred more often. In little gull (L. minutus), trematodes were single specimens. Mature A. müehlingi are registered only in these gull species during the spring period, immediately after their return from the wintering areas. Detection of the parasite indicates a potential environmental threat. To date, A. müehlingi is found only in the definitive host. However, the penetration of gastropods Lithoglyphus naticoides Pfeiffer, 1828, the parasite's first intermediate host, into Lake Ladoga will form favorable conditions for the life cycle of the parasite. This may lead to the epizooty of fish by A. müehlingi.