Juvenile Fasciola hepatica 12, 14, 18, 20, and 24 days old were successfully transferred to normal recipient mice. Worms entered the common bile ducts when total worm age was 30 or 32 days. Liver migration time was controlled between 16 to 17 and 3 to 4 days. Flukes 12, 14, 16, 18, 20, and 24 days old were transferred to normal recipients in an attempt to relate the duration of liver migration and fluke age to the stimulation of acquired immunity. When transferred worms had a total age of 40 days immunized mice were given a challenge infection of 2 metacercariae per mouse. At 25 days after challenge these mice were killed and worms were recovered. Immunization with 12-, 14-, 16-, and 18-day-old worms produced a significant reduction in challenge worm burdens when compared to natural immunity controls. Immunization with 20- and 24-day-old worms did not stimulate a significant acquired immunity. It is suggested that the duration of liver migration, at least 10 to 11 days, by young worms and not the specific age of the young worm is responsible for the stimulation of acquired immunity.