Twenty-three turtles representing 3 families (5 species)—Chelydridae: common snapping turtle, Chelydra serpentina; Emydidae: brown wood turtle, Rhinoclemmys annulata; painted wood turtle, Rhinoclemmys pulcherrima; Kinosternidae: white-lipped mud turtle, Kinosternon leucostomum; scorpion mud turtle, Kinosternon scorpioides—collected in the Area de Conservación Guanacaste, Costa Rica, were subjected to postmortem examination. Gravid nematodes representing 10 species, 9 previously described—Atractis costaricaensis, Cosmocercoides variabilis, Dracunculus globocephalus, Falcaustra chelydrae, Falcaustra tikasinghi, Klossinemella caballeroi, Sauricola sauricola, Serpinema magathi, Spiroxys figueiredoi—were found. Eight new host records and 7 new locality records are reported. A new species of Falcaustra inhabiting Rhinoclemmys annulata is described; it is the only known member of the genus lacking a pseudosucker, but having a toothed chamber separating the vestibule and the pharynx. Only Falcaustra kinsellai, infecting Heosemys grandis from Malaysia, has a toothed chamber separating the vestibule from the pharynx, but F. kinsellai has a pseudosucker.