Metacercariae of Fasciola hepatica were excysted in a simple system based on the stimuli determined by Dixon (1966). Reproducibly high levels of excystment (70-80%) were obtained within 3h. Equal volumes of human serum and medium RPMI 1640 with 2% washed human red blood cells supported better growth in vitro than human serum diluted with ELac or NCTC 135. Reduced rates of growth were observed with serum concentrations lower than 50%. During culture over a period of 14 weeks some organisms in every culture grew to a length of 3 mm at a linear rate approximately one quarter of the growth rate in vivo (mouse). A few parasites suddenly began to develop more rapidly after six weeks in culture and reached 6-7 mm in length, comparable to the size of sexually mature Fasciola grown in mice. These cultured worms showed extensive development of the uterus, vitellaria, and testes with spermatozoa. The ovary remained rudimentary and egg formation did not occur.