Bonamiasis, first detected in the New Zealand dredge oyster Tiostrea lutaria (Hutton) during the southern autumn of 1986 was investigated through 2 surveys of oyster beds carried out over spring (Sep 1986) and summer (Jan 1987). In spring, a 'lighter' form of the parasite occurred most frequently, confined largely to the sub-epithelial tissue. In summer, a 'dense' form, nearly identical to the 'classic' European and American Bonamia ostreae, was the dominant type; it occurred in sub-surface tissue around the digestive diverticula and gut, and invaded gonadal follicles and gill filaments. Other forms, such as a second type of the 'dense form' and what appeared to be intermediate stages between the 'lighter' and 'dense' types were also detected. The significance of these different types in the life cycle of the parasite is discussed. The New Zealand species shows distinct size and morphological differences from B. ostreae.