Fish are important hosts for many parasites that cause them various damage, such as weight loss, reduced mobility, and an inability to escape predators, as well as affecting reproduction when their gonads are infected with parasites. Ectoparasites that infect the skin, mouth, gills of fish can negatively affect their health by interfering with respiration. One such parasite is Gnathia sp., which infects many species of fish, including Epinephelus costae in the marine waters off the coast of Sirte in Libya. This study aims to investigate Gnathia sp., a species of isopod parasite that infests the mouth and gills of Epinephelus costae fish, and to isolate and describe this parasite. In this study, 50 samples of Epinephelus costae were collected from the marine waters off Sirte, Libya, between December 2023 and February 2024. The fish were examined externally, as well as in the mouth and gills. The results showed that 24 fish were infected with the Gnathia sp, parasite. Six fish were infected only in the mouth, four in the gills, and 14 in both the mouth and gills. This study on the Gnathia sp. parasite represents a significant achievement in the field of research, as it is the inaugural study to be conducted in the city of Sirte, Libya, and the inaugural documentation of this parasite in the study area............. Keywords: ............Epinephelus costae, Ectoparasites, Gnathia sp.