Abstract.A hitherto unrecorded haplosporean was found in the hepatopancreas ofPenaeus vannamei. Multinucleate plasmodia developing within the hepatocytes fragment into unimicleate stages. Eventually the host cells are destroyed and the uninuclear stages are released into the lumen of the secretory tubules. The haplosporosomes are oval, have a thick cortex and contain a small spherical medulla at the broader end. No spores were found and thus the parasite, which is highly pathogenic, could not be assigned any precise taxonomic position.