The most common endoparasite encountered was the nematode, _Mastophorus numidica_ Seurat, 1914; it was found in the stomachs of many individuals of both species of _Peromyscus_. This nematode has been reported from _Felis ocreata_ in Algeria, _Bitis arietans_ in the Congo, and from the following mammals in the United States: _Canis latrans_, _Peromyscus crinitus_, _P. gossypinus_, _P. maniculatus_, _P. truei_, _Onychomys leucogaster_, _Dipodomys ordii_, _Reithrodontomys megalotis_, and _Eutamias minimus_. Individuals of _P. maniculatus_ obtained on the northern end of Wetherill Mesa in May and June of 1962 had numerous ectoparasites. At this time, the population of _P. maniculatus_ was high, but on a downward trend. My data and observations lead me to conclude that individuals of _P. maniculatus_ are more heavily parasitized by both botflies and endoparasites than are individuals of _P. truei_. The reasons for this unequal amount of parasitism in two species of mice occurring in the same general area remain obscure. The kinds of endoparasites and ectoparasites collected from _P. maniculatus_ and from _P. truei_ are listed below (m = present in _P. maniculatus_, t = present in _P. truei_). ACARINA: Ixodidae: _Dermacentor andersoni_ mt, _Ixodes angustus_ mt, _Ixodes spinipalpis_ m. Laelaptidae: _Androlaelaps glasgowi_ m. Myobiidae: _Blarinobia_ sp. m. Trombiculidae: _Euschoengastia lanei_ mt, _Euschoengastia criceticola_ m, _Euschoengastia dicipiens_ t, _Euschoengastia peromysci_ m, _Leewenhoekia americana_ m, _Trombicula loomisi_ m. DIPTERA: Cuterebridae: _Cuterebra cyanella_ mt. SIPHONAPTERA: _Callistopsyllus deuterus_ m, _Catallagia decipiens_ m, _Epetedia stanfordi_ mt, _Malaraeus sinomus_ mt, _Malaraeus telchinum_ mt, _Megarthroglossus procus_ mt, _Monopsyllus wagneri wagneri_ mt, _Orchopeas leucopus_ mt, _Peromyscopsylla hesperomys adelpha_ mt, _Phalacropsylla allos_ t, _Rhadinopsylla sectilis goodi_ t, _Stenistomera macrodactyla_ m, _Stenoponia_ (_ponera_ or _americana_) mt. CESTODA: _Choanotaenia_ sp. m, _Hymenolepis_ sp. t. NEMATODA: _Mastophorus numidica_ mt, _Syphacia obvelata_ mt, _Trichuris stansburyi_ t. ACANTHOCEPHALA: _Moniliformis clarki_ mt. PREDATION