A new nematode species, Philometra margolisi n.sp., is described from the gonads of the serranid fish Epinephelus morio from Mexico (localities Telchac, Chuburna, Progreso, and Rio Lagartos, Yucatan). It is characterized mainly by long (0.432-0.468 mm) and slender spicules and a simple gubernaculum (length 0.084-0.093 mm) in the male and by body shape and size (length 65-85 mm), arrangement and number of small cephalic papillae (eight papillae in an outer circle and four in an inner one), structure and length (1.07-1.18 mm) of the esophagus, long ovaries, and the presence of a pair of small lateral papilla-like projections on the elongate caudal end in gravid females.