The effect of ageing of adults of Echinostoma friedi (Trematoda: Echinostomatidae) on the infectivity of miracidia yielded was analysed. Miracidia were obtained after hatching of eggs obtained from adult worms of E. friedi collected weekly during the course of experimental infections in golden hamsters. Miracidial infectivity, measured in terms of percentage of infection in Lymnaea peregra, was significantly influenced by the age of the adult worms from which the miracidia were derived. Infective miracidia only were obtained from adult worms in the age range from 4 to 9 weeks post-infection. Infectivity was maximal in those miracidia derived from adults collected 8 and 9 weeks post-infection. The results suggest that adult worms producing viable eggs require additional maturation to be able to yield eggs containing infective miracidia.