Survival and infectivity characteristics are described for cercariae of the echinostome Echinoparyphium recurvatum at 10, 15, 20, 25 and 30°C. Cercarial survival was markedly age- and temperature-dependent, maximum survival time being reduced from 68 h at 10°C to 12 h at 30°C, 50% survival times falling from 47·9 h at 10°C to 8·4 h at 30°C. The mean instantaneous per capita death rate of cercariae increased exponentially with temperature. Cercarial infectivity was also temperature dependent, the main effect of temperature being upon the rate at which infectivity diminished with increasing age of cercariae. Infectivity increased from zero at 10°C to a maximum at 25°C after which it declined to a low level at 30°C. Overall transmission efficiency of cercariae at each experimental temperature was calculated as the ratio of the mean instantaneous per capita death rate (μ) to the mean instantaneous per capita rate of infection (β). Transmission efficiency of cercariae was found to be maximal at 20°C and at a relatively high level between 10 and 25°C. Transmission efficiency was zero at 10°C and at a very low level at 30°C. These results suggest that the cercariae of E. recurvatum show transmission optima at water temperatures likely to be encountered in natural habitats in Britain and Europe.