Detailed examination was carried out on 1745 oysters (Tiostrea chilensis syn. T. lutaria) sampled from two pairs of stations, 15 km apart, in Foveaux Strait between Sep 1986 and May 1989. Occurrence, intensity of infection and parasite stages were recorded, to determine the annual pattern of infection. A consistent pattern was apparent at both station pairs over the 3 years. Infections were lowest in spring (Oct/Nov). Rapid proliferation occurred in early summer (Dec/Jan). Gonad infections with larger dense forms persisted through the mainly female cycle. In early winter (May/Jun) infection rates declined, but a second peak in prevalence occurred in winter (Aug). Egress from the host was effected primarily through the gonad (Dec-Aug), kidney (Apr-Aug), digestive diverticulate and gill (Jan-Aug), and presumably on the death of the host. Parasite tissue distribution was related to crowding except in the male gonad where it coincided with haemocyte phagocytosis of gonad products.