Abstract A new nematode species, Cucullanus oaxaquensis n. sp., is described from specimens recovered from the intestine of Thorichthys helleri Steindachner, 1864 from the Ró Modelo, Los Chimalapas, state of Oaxaca, Mexico. It is characterized largely by possesing unusual features among Cucullanus spp., i.e., net-like sculpture on the surface of eggs, a pair of small papillae near tail tip of females and a small terminal sclerotized structure at tail tip of both males and females. This findig represents the fouth species reported from freshwater fishes in Mexico and the second for cichlid fishes.