Seventy-eight snakes collected in the Area de Conservación Guanacaste, Costa Rica were necropsied: Boidae (1 species), boa constrictor, Boa constrictor; Colubridae (21 species), Ecuador sipo, Chironius grandisquamis; road guarder, Conophis lineatus; South American forest racer, Dendrophidion percarinatus; speckled racer, Drymobius margaritiferus; mimic false coral snake, Erythrolamprus mimus; blunthead tree snake, Imantodes cenchoa; western tree snake, Imantodes inornatus; banded cat-eyed snake, Leptodeira annulata; striped lowland snake, Leptodrymus pulcherrimus; Cope's parrot snake, Leptophis depressirostris; yellow-bellied racer, Liophis epinephelus; neotropical whip snake, Masticophis mentovarius; brown vinesnake, Oxybelis aeneus; Cope's vine snake, Oxybelis brevirostris; green vine snake, Oxybelis fulgidus; Cope's false coral snake, Pliocercus euryzonus; cloudy snail-eating snake, Sibon nebulata; chicken snake, Spilotes pullatus; western lyre snake, Trimorphodon biscutatus; orange-bellied littersnake, Urotheca guentheri; false fer-de-lance, Xenodon rabdocephalus; Viperidae (6 species), cantil, Agkistrodon bilineatus; jumping pit viper, Atropoides nummifer; terciopelo, Bothrops asper; cascabel, Crotalus durissus; hognosed pit viper, Porthidium nasutum; and slender hognose viper, Porthidium ophryomegas. Gravid nematodes representing 16 known species, Aplectana incerta, Aplectana itzocanensis, Cosmocercoides variabilis, Cruzia rudolphi, Hastospiculum onchocercum, Hexametra boddaertii, Kalicephalus costatus, Kalicephalus inermis, Kalicephalus sublatus, Macdonaldius oscheri, Ophidascaris arndti, Ophidascaris sicki, Physaloptera retusa, Skrjabinelazia intermedia, Terranova caballeroi, Travassosascaris araujoi, 1 new species, Abbreviata costaricae, and larvae of a species of Porrocaecum were found. Twenty-six new host records and 4 new locality records are reported. The nematode species found in this study are generalist parasites, i.e., infect more than one host species, and a comprehensive review of the hosts of these species is presented.