ABSTRACT: Eosinophilic intranuclear inclusions are reported from agranular haemocytes and eosinophilic granulocytes of Ostrea angasi (Sowerby, 1871). Ultrastructurally, ovoid to hexagonal capsids 98 ± 4 nm (range 90 to 105 nm; n = 60) in diameter formed around an ovoid, fine granular inclusion at the periphery of the nucleus. Strands and rod-like configurations of dark material near the centre of the nucleus developed into a coarse granular matrix along the edge of which capsids acquired a fibrillar, vermiform or rod-like core. Many capsids appeared to degenerate along the nuclear periphery. Those passing into the cytoplasm acquired an envelope in the perinuclear space, and cytoplasmic nucleocapsids acquired an envelope when passing into endoplasmic reticulum or cytoplasmic vesicles. The few virions observed had tails and were 135 to 140 nm across. Nuclear stages of replication are more similar to cytomegaloviruses (Betaherpesvirinae) than to herpes simplex viruses (Alphaherpesvirinae). Incomplete replication and the low level of virion production are attributed to sampling in late summer and winter when temperatures are sub-optimal.