The growth rates of the intramolluscan stages of the parasite Fasciola hepatica were determined at three temperatures 16°, 20° and 25°C. A graphical relationship of growth rate with respect to temperature was produced and a growth array was calculated from this. Meteorological data were summarized in the form of a temperature array. These arrays were then used in conjunction with a simple equation to simulate parasite growth within the snail host. A digital computer was employed to solve iteratively the simulation equation, initially by checking its application against experimentally determined results. It was then employed with the meteorological data to simulate growth with respect to environmental temperatures and the results obtained were compared with those derived from field collections. Estimates were also made of the duration of the shortest life-cycle under field conditions.