The migration of Trichobilharzia ocellata schistosomula in the duck host and the accidental mouse host was studied by the detection of [75Se]-methionine in radiolabelled cercariae using autoradiography of compressed host tissues and by the recovery of schistosomula following the mincing and incubation of host tissues. Exposure via duck feet (30 min) and mouse tails (60 min) resulted in low infection rates: 3.9% in the duck and 3.1% in the mouse. However, when mice were infected via the abdominal skin using the ring method, 25% of the cercariae were recovered in the skin. In the duck, schistosomula left the skin within 3 days post-infection (p.i.) and were detected in the lungs between days 2 and 4 p.i. Thereafter, only few radioactive foci were found in the liver, kidney and intestine. In the mouse, schistosomula were detected in the lungs within 10 h and for up to 6 days p.i., and some foci were detected in the liver and intestine on days 2-5 p.i. On days 3-4 p.i., 36% of the detectable parasites in the mouse (91 foci in 10 mice) had migrated from the skin to other organs. Living schistosomula could be extracted from mouse skin and lungs only until day 4 p.i. The schistosomula of T. ocellata migrate faster within their vertebrate hosts than do those of Schistosoma mansoni, indicating that the former may use other transformation and immune-evasion mechanisms.