A new enteric microsporidian was found to be associated with the mass mortality of hatcherybred juvenile groupers, Epinephelus spp., in China. The outbreak usually occurred during the rainy season between May and November when water temperature ranged from 26 to 30 degrees C and salinity from 28 to 34 ppt, although this microsporidian can be detected year round. External clinical signs included severe emaciation, white faeces syndrome, anorexia, sinking to the bottom of culture ponds and mass mortality (up to 90%). Upon necropsy, severe intestinal oedema and thin and transparent intestinal wall could be observed. The mature spores are tiny, measuring 1.3-1.5 (1.35 +/- 0.13) x 1.6-2.4 (2.16 +/- 0.31) mu m and can be found in the cytoplasm and the nucleoplasm of most enteric epithelial cells of host. Epidemiological investigation showed that this species was distributed throughout most of the culture area of grouper fingerlings in Fujian, Guangdong, Hainan and Guangxi provinces in China, with maximum prevalence of 95%. Molecular analysis based on the partial small subunit rRNA sequence (1045 bp) placed this species within the Enterocytozoonidae, but sequence identities to other species were below 90%. The exact taxonomic position warrants study of the ultrastructural characteristics of the developmental stages. Keywords: emaciation, enteric microsporidiosis, groupers, mass mortality, white faeces syndrome.