The region of the Sierra San Antonio Peña Nevada is considered a priority terrestrial area for conservation. Studies of its flora and fauna are scarce, especially with regard to host-parasite relationships. The aim of this study was to determine the host-parasite relationship between small mammals and fleas and ticks, because of the lack of taxonomic studies of this relationship. Small mammals were collected with Sherman type traps during a period since February 2000 to December 2002. These animals were anesthetized with chloroform and then ectoparasites were separated. A total of nine species of small mammals were collected Liomys irroratus, Neotoma goldmani, Neotoma mexicana, Peromyscus boylii, Peromyscus maniculatus, Microtus mexicanus; Mustela frenata; Cratogeomys castanops, and Thomomys umbrinus; on which six species of ectoparasites were found, corresponding Dermacentor albipictus, Ixodes sp., Argas sp., Hoplopsyllus affinis, Stenoponia americana and Nosopsyllus fasciatus. The present is the first study in SSAPN about the host-parasite relationship.