A polyopithocotylean monogenean Gotocotyla secunda (Tripathi, 1956), is considered one of the negatively influencing parasitism on the gills of Scomberomorus commerson Lacepède 1801 inhabiting the Mediterranean Sea near Damietta City, Egypt has been investigated in this study. The light and scanning electron microscope studies revealed that the parasites attached themselves by grasping one or two tips of the secondary gill lamellae of S. commerson and with its elongated body located and extend at the same line to the primary gill lamellae with its head protrudes towards the tips of the primary gill lamellae. Result detects the harmful effect of G. secunda, that comprise of the gill lamellae, causing several damages to primary and secondary gill lamellae. These effects cause curling up, erosions, and atrophy of secondary gill lamellae, damaged central core in secondary gill lamellae, secretions of mucus, hemorrhage of blood vessels, vacuole formation and lamellar, hyperplasia of epithelial cells, necrosis of gill filaments. In addition to the secondary gill lamellae of the gill filament as well as of the neighboring gill lamellae were found to be compressed.