A Minchinia sp. (Haplosporidia: Haplosporidiidae) parasite was identified infecting rock oysters and morphologicallydescribed by Hine and Thorne (2002) using light microscopy and transmission electron microscopy (TEM). The parasitewas associated with up to 80% mortality in the host species and it is suspected that the parasite would be a majorimpediment to the development of a tropical rock oyster aquaculture industry in northern Western Australia. However,attempts to identify the parasite following the development of a specific probe for Haplosporidium nelsoni were unsuccessful.The SSU region of the parasite's rRNA gene was later characterized in our laboratory and an in situ hybridization assay forthe parasite was developed. This study names the parasite as Minchinia occulta n sp. and morphologically describes theparasite using histology, scanning electron microscopy and transmission electron microscopy. The non-spore stages were unusual in that they consisted primarily of uninucleate stages reminiscent of Bonamia spp. The parasite's spores were ovoidto circular shaped and measured 4.5 mm'5.0 mmr3.5'4.1 mm in size. The nucleus of the sporoplasm measured 1.5'2.3 mmand was centrally located. The spores were covered in a branching network of microtubule-like structures that may degradeas the spore matures.