A new monogcncan species, Pseudorhabdosynochus capurroi sp. n., is described from the gills of the black grouper, Mycteroperca bonaci (Poey) (Serranidae), from the Yucatan Peninsula, Mexico. The new species can be differentiated from all other known species of Pseudorhabdosynochus Yamaguti, 1958 by being the largest one recorded until now (1162-1535 pm body length). Moreover, the vagina of P. capurroi has a small funnel-shaped opening with a non sclerotized vesicle from which a fine sclerotized duct leads proximally to a sclerotized seminal receptacle, and the testis is bilobate. The squamodiscs have 14 to 16 rows of rods, of which 3 to 5 rows are closed.
Sciadicleithrum meekii sp. n. is described from the gills of the cichlid fish Cichlasoma meeki (Brind) from cenotes (= sinkholes) of the Yucatan Peninsula, Mcxico.The new species differs from congeners by having a dorsal hamuli with a prominent superficial root articulated with a straight shaft and curved point, and a vagina with a coiled tube comprised of one ring.