A Giant groupers (Epinephelus lanceolatus) farm in southern Taiwan displayed symptoms of hypoxia with mortality around 3.5%. Wet mount examination revealed mottled gills with abundant oval-shaped cysts as the main characteristic lesions. Sporadic occurrence of diffusely distributed white nodules in the kidney was also observed. Through histopathological examination, basophilic, cytoplasmic cysts were found diffusely scattered in the secondary lamellae. The contents of the cysts produced a dark magenta hue under Macchiavello stain, indicating the presence of chlamydia-like organisms. Focal to multifocal granulomatous, necrotic lesions were observed in the kidney and spleen, and were the result of Vibrio spp. and Photobacterium spp. infection. DNA samples were extracted from the gills and the resulting product was a 298 bp fragment with a nucleotide sequence that was 98% homologous with Simkaniaceae spp. As the result, the final diagnosis was epitheliocystis with secondary bacterial infection.

Key words: Epitheliocystis, giant groupers (Epinephelus lanceolatus), Macchiavello stain, Simkaniaceae spp, Taiwan