The pathology associated with an intracellular ciliate infection in the digestive gland of pearl oysters Pinctada maxima (Jameson, 1901) is described. Histopathological and transmission electron microscopic examination were used to characterise the organism and its location within host cells. The parasite is tear-drop shaped measuring 5.53 'm (range of 2.73'7.47 'm, n = 9) in width and 11.15 'm (range of 9.02'16.2 'm) in length with a centrally located lobulated nucleus and a large nucleus:cytoplasmic ratio. The ciliate has nine evenly spaced rows of cilia running obliquely along the length of cell, converging on the pointed end. Infected digestive glands typically had a moderate to severe infiltration with mononuclear hemocyte. A strong correlation existed between the burden of ciliates and the host response; (p < 0.001, C = 0.315 Pearson Correlation). The use of a single tissue section upon microscopic examination was found to detect only 38'50% of the infections. However, examination of serial haematoxylin and eosin stained sections improved the reliability of detecting infection.