The number of larvae in the Cosmocerca ornata (Dujardin, 1845) females from marsh frogs in vivo and period of larvae production of nematodes in vitro was studied. The number of larvae in females of nematodes and quantity of born larvae depend on the size of parasites. The largest C. ornata females had the highest number of larvae and the longest period of larvae production. The nematode sizes in its turn depends on density of infrapopulation of C. ornata and ambient temperature. The dependence of a number of the C. ornata larvae on the host age and sex and on a season of the year was revealed. Different phenotypes of amphibians showed no influence on the number of larvae inside nematode females. In the experiment C. ornata females remained viable up to 8 days, producing larvae up to 7 days. The optimum temperature for the larvae production constituted 24—28 °C. At a temperature of 12 °C and lower the nematode larvae output from females was stopped.