Pseudorhabdosynochus yucatanensis sp. n. (Diplectanidae) is described from the gills of the red grouper, Epinephelus morio (Valenciennes, 1824) (Serranidae), from the coast of the Yucatan Peninsula, Mexico. This new species is characterized by having both the ventral and the dorsal squamodiscs composed of 10-12 rows of rods, with 0-1 innermost row forming a closed circle. Furthermore, the vagina of P. yucatanensis has a non-sclerotized ampulla with a fine, sclerotized duct leading to a sclerotized seminal receptacle. Prevalence (percentage of infected fish) and abundance (mean number of worms per examined fish) were estimated for P. yucatanensis from 8 localities along the coast of Yucatan. Prevalence varies from 38% to 100%, while abundance was between 1.2 +/- 0.6 and 43.2 +/- 17.8 worms per fish. The lack of linear relationship between the host length and the number of monogeneans per fish (regression F1,188 = 0.56, p = 0.45) was probably due to the fact that the sample was restricted to juvenile hosts.