Experimental infestation of the marsh frog tadpoles with the Cosmocerca ornata (Du- jardin, 1845) larvae was conducted. In vitro development of the first, second and third larval stages of the helminth were studied. Tadpoles of different age were infested both i) by feeding larvae to them and ii) by placing them in containers simultaneously with the C. ornata larvae. In both cases dissection revealed the presence of living larvae of the nematode in the tadpoles' intestines. Despite their motility and activity, the I and II larval stages were not invasive; they never persisted inside the tadpoles' digestive tract. In 4-5 days after the second molt, a "temporary persistence" was observed. Living larvae were in suppressed condition, motility lowered, further development never occurred. In 3 days occurrence of the III stage nematode larvae lowered drastically. Gradual elimination of the most part of C. ornata larvae was observed. Elimination rate depended on the tadpole developmental stage. Later tadpole stages (those that had already developed arms and legs) were the first to get rid of the nematode larvae--on 7-th day after the infestation. On the 9-th day tadpoles of the early stage (those without limbs) got rid of the larvae. Larvae persisted for the longest time in the intestines of middle-staged tadpoles (with the hind legs)--up to 13 days.