Field trials were carried out in north Devon to investigate the relationship between molluscicide treatment of pastures and control of liver fluke infection in sheep. Seven tracer lambs per plot were used to estimate the infectivity of 17 pairs of 0.20 hectare plots. One plot in each pair was treated with the molluscicide Frescon (N-tritylmorpholine). There was a highly significant difference (P less than 0.001) between the numbers of Fasciola hepatica recovered from lambs grazed on treated and untreated plots in the period after molluscicide treatment. The overall degree of snail control achieved by one application of molluscicide to 17 plots was about 90 per cent and this was matched by a comparable degree of liver fluke control. There was considerable variation between the plots and there was no simple correlation between snail numbers and liver fluke numbers (r = -0.03). The use of molluscicides is discussed in relation to the biotic potential of Lymnaea truncatula and environmental factors which limit its population growth.