Two species of chewing lice (Eutrichophilus mexicanus and Neotrichodectes mephiditis), 4 species of sucking lice (Fahrenholzia fairchildi, Fahrenholzia ferrisi, Fahrenholzia n. sp., and Hoplopleura mendezi), 4 species of fleas (Kohlsia traubi, Polygenis roberti beebei, Rhopalopsyllus australis australis, and Rhopalopsyllus lugubris), and 1 species of epifaunistic pseudoscorpion (Epichernes guanacastensis) were recorded from native mammals in the Área de Conservación Guanacaste in northwestern Costa Rica during a parasite All Taxa Biodiversity Inventory from 1996 to 2004. All 6 species of lice were host specific. Fleas were less host specific and included a record of a rodent flea (R. a. australis) from a jaguar, Panthera onca. Epifaunistic pseudoscorpions were recorded only from Salvin’s spiny pocket mouse, Liomys salvini, on which they appear to be mutualistic with the host by preying on ectoparasites in the host pelage. Eutrichophilus mexicanis, N. mephiditis, F. ferrisi, H. mendezi, K. traubi, and R. a. australis are reported from Costa Rica for the first time.