The life cycle of the nematode Cosmocerca ornata (Dujardin, 1845)—a parasite of amphibians—was studied for the first time. The experimental infection of yearlings of the marsh frog Pelophylax ridibundus infected by larvae of C. ornata was performed. The development stages of parasites in the host were studied. It has been revealed that amphibians are infected with infectious larvae (J3) of nematodes in the surface-water layer by the penetration of parasites through eyes onto the conjunctiva under the lower eyelid, where the larvae undergo the third and fourth molting. Then the parasites migrate into the oropharyngeal cavity and through the digestive tract to the place of constant localization: the posterior part of the intestine. Morphological descriptions and original drawings of J3 and J4 juveniles, molting juveniles, and adults of C. ornata are given. The development stages of the juveniles are diagnosed by the body size, location of the genital primordium, and forming copulative structure. The sex of the nematodes can be identified at the third molt stage. Mature nematodes differ from juveniles by the presence of copulative organs: the vulva in adult females and spicules in adult males. The time of the development stages of C. ornata and the total life span of the parasite in the host is determined. The effect of ambient temperature on the period of development of nematodes in amphibians is established.