The effect of host variables such as size and density, on the transmission of cercariae of Diplostomum spathaceum (Rudolphi, 1819) into a second intermediate fish host, rainbow trout, Oncorhynchus mykiss (Walbaum), was studied quantitatively in laboratory experiments under varying exposure conditions. Differences in the number of established metacercariae were noted in differently sized fish exposed singly to the same number of cercariae and in water volumes related to the body surface area of the host. When exposed (1) singly in an equal water volume or, (2) simultaneously in the same tank, no differences in recovery of metacercariae per fish were found between “small” and “large" hosts. The latter observation is valid for three water volumes tested using the same number of cercariae per host. No significant difference in metacercarial infection was revealed in similar sized fish exposed at different host densities. It appears that transmission is more influenced by cercarial density (number of cercariae per water volume) and fish size than by fish density. These experimental findings support the view that cercarial infection offish occurs by chance, presumably mainly in the gill region. Accordingly, in the field, individual fish size and cercarial density rather than fish population density, prevail in parasite transmission from snail to fish. This is of interest in coastal areas heated by cooling water, where fish growth and fish population density are enhanced.