The development of Trichobilharzia australis Blair & Islam, 1983 in the intermediate host, Lymnaea lessoni Deshayes and in experimental definitive hosts, Muscovy ducks is described, 24 hours after entry into the snail, miracidia had lost their cilia, epidermal plates and lateral processes and became young mother sporocysts. They were located in the tissues of the head-foot organ of the snail and were oval in cross section but still retained the shape of a miracidium, and measured 0·058–0·66 × 0·041–0·049 mm. From the seventh day onward young mother sporocyts were tubular, thin-walled, irregular in shape and were in the tissues of the lung and kidney of the snail. On the 24th day mature mother sporocysts and young daughter sporocysts were found in the digestive gland. Between the 24 th and 29th day mature daughter sporocysts with fully developed cercariae ready to emerge, or already emerged, could be seen in the digestive gland of the snail. Cercariae emerged from the snail from the 29th to 46th day after exposure at 25°±1°:C. The prepatent period of T. australis in the Muscovy ducks was 22 to 42 days after the beginning of exposure to cercariae.