Distribution and abundance of seven species of ectoparasitic mites of the genus Gigantolaelaps Fonseca were examined during a 14-mo intensive study of the small-mammal fauna in two adjacent ecological reserves near Brasilia, Federal District, Brazil. Prevalence (extensity) and intensity of infestation were combined with measures of reproductive capacity of mite infrapopulations to estimate specificity of observed host/mite associations. Gigantolaelaps spp. occurred almost exclusively on the oryzomyine (Cricetidae: Sigmodontinae: Oryzomyini) component of the small-mammal fauna. Within this host group, mite associations were species specific. Each of seven oryzomyine species was infested consistently with a different species of Gigantolaelaps, and evidence of mite reproduction was observed only in these association pairs. The host/mite pairs are Gigantolaelaps amazonae Furman on Oryzomys bicolor (Tomes), Gigantolaelaps oudemansi Fonseca on Oryzomys capito (Olfers), Gigantolaelaps guimaraesi Lizaso on Oryzomys concolor (Wagner), Gigantolaelaps peruviana (Ewing) on Oryzomys fornesi Massoia, Gigantolaelaps wolffsohni (Oudemans) on Oryzomys nigripes (Olfers), Gigantolaelaps vitzthumi Fonseca on Oryzomys subflavus (Wagner), and Gigantolaelaps goyanensis Fonseca on Nectomys squamipes (Brants). Oryzomyine rodents showed strong preferences for certain macrohabitats; five species occurred exclusively in gallery forest and two in brejos, upland campos, and cerrados. Secondary infestations of Gigantolaelaps spp. on atypical hosts were rarely observed and only in the gallery forest macrohabitat ( G. oudemansi and G. goyanensis on Didelphis albiventris Lund, and G. oudemansi on O. concolor). Reproductive mites were not observed on secondary hosts.