Using an indirect immunofluorescence technique, sera from 113 cynomolgus monkeys (Macaca fascicularis), trapped in Peninsular Malaysia, were screened for the presence of antibody to six prototype strains of Rickettsia tsutsugamushi combined into three polyvalent groupings: I--Karp, TA716, and TA763; II--Gilliam; and III--TA678 and TH1817. Fifteen percent (17/113) of the monkeys had antibody titers greater than or equal to 1:50 to one or more of the antigenic groups. Although a titer greater than or equal to 1:150 is generally considered indicative or prior Rickettsia tsutsugamushi infection, we selected a less than 1:25 titer as a conservative standard to insure non-infected animals. Using this criterion, 62 (55%) of the 113 monkeys were accepted for use in scrub typhus studies. The high prevalence of antibody to scrub typhus in the semi-arboreal cynomolgus monkey is in marked contrast to the low prevalence reported in the strictly arboreal silvered leaf monkeys (Presbytis cristatus). The results of this study indicate that cynomolgus monkeys should be rigorously screened for evidence of prior infection before they are included in experimental scrub typhus studies.