Rats and mice were infected with either normal metacercariae or metacercariae γ-irradiated at 3 krad. or 4 krad. Flukes were recovered at various times after infection and their teguments examined using a transmission electron microscope. In normal flukes, the secretory granules TO, Tl and T2 were all seen during tegumental development. The teguments of flukes from mice developed faster than the corresponding teguments in rats. TO granules were present from day 0 to day 10 post-infection (p.i.) in mouse flukes and from day 0 to day 14 p.i. in rat flukes. Tl granules first appeared in mouse flukes by day 4 p.i. but not until day 8 p.i. in rat flukes. T2 granules were seen in mouse flukes 2 days p.i. but not before 6 days p.i. in rat flukes. γ-Irradiation at 4 krad prevented normal tegumental development in flukes from both rats and mice. TO granules were present at all times in flukes from either host. Tl granules were produced in mouse flukes but their appearance was delayed until day 6 p.i. No significant production of T2 granules occurred in flukes from either host. Parasite survival was also affected by γ-irradiation and none of the flukes reached maturity. Flukes from rats died between 10 and 21 days p.i and flukes from mice died between 14 and 28 days p.i. γ-Irradiation of metacercariae at 3 krad. had an extremely variable effect on subsequent tegumental development in both rats and mice. Some flukes developed normally, some showed development associated with γ-irradiation at 4 krad, whilst some showed intermediate development.