Gonapodasmius epinepheli, a didymozoid trematode, was found in orange spotted grouper (Epinephelus coioides, Hamilton-Buchanan, 1822), in Khark Island coastal area in Persian Gulf, Iran. The Gonapodasmius epinepheli encysted on gills in primary lamellae. Histopathological serial sections revealed that larvae were mainly embedded under the gill epithelium and eggs of the parasite were disseminated in some organs such as gill epithelium and uterus lumen. Pathological changes and host response were primary reduction and destruction of secondary lamellae. Various stages of the life cycle of this parasite were discussed in this study.