The reproductive structure of Cosmocerca ornata (Dujardin, 1845) in marsh frogs (Pelophylax ridibundus (Pallas, 1771)) of both the striata and non-striata phenotypes was studied. Distinctions in the physiology, ecology and behavior of striped and unstriped amphibians have certain influence on the intensity of processes of the host infestation and maturing of C. ornata in frogs of different phenotypes. Analysis of the age structure of the C.