Quantitative observations on the growth rate of vegetatively propagated strains of larval Echinococcus multilocularis sibiricensis in cotton rats and white mice are reported, and some peculiarities of growth in these animals are discussed. It is shown that the weight of 70-day-old cysts of the seventh transfer in cotton rats was about 25% of the total weight of the host animals, and that the fast growth and the elimination of the hazardous work with definitive hosts make this strain suitable for chemotherapeutic studies.