The growth of larval Echinococcus multilocularis in cotton rats, jirds, and A/J mice was measured. Single, thin-walled, acephalic cysts from cotton rat donors were inoculated intraperitoneally into the experimental hosts, and cyst weights were taken at several intervals after infection. It is shown that larval growth can be quantitatively described since it follows a sigmoid curve. Protoscoleces were produced within the cysts in all three hosts, although the time of appearance differed in each case.