An epidemiological survey of the distribution and abundance of Diplostomum spathaceum in farmed rainbow trout (Salmo gairdneri) from Mill of Cantray trout farm (Nairn, Morayshire) involved monthly sampling of fish over a 3-year period. Diplostomum metacercariae were present in both the lens and aqueous humour of infected fish, and these have been treated separately throughout the study. The infection period was normally between May and September each year and transmission of the parasite from snail to fish did not occur at temperatures below 10 degrees C. The prevalence and abundance of both lens and humour metacercariae reached a maximum in September. The cleansing and application of a molluscicide (copper sulphate) to the raceways in spring resulted in a 56% reduction in the numbers of metacercariae infecting trout during the following summer. However, no further improvement in parasite control was recorded when the treatment was repeated in the following year. Experiments using caged fish indicated that diplostomiasis was confined to certain areas of the farm only and that the infection rate of rainbow trout with D. spathaceum cercariae was correlated (P less than 0.01) with water temperature. The results of the study indicated that it is possible by regular cleaning and use of molluscicides to keep the intensity of diplostomiasis at such a level that rainbow trout do not become severely affected. However, as with other parasitic diseases, a combination of control methods will probably be required to eradicate the disease completely from trout farms.