The diverse biogeography of Chile provides it an ideal field to study the influence of environmental factors on plant and animal biodiversity. Some small animal species are distributed throughout dissimilar environmental zones, such as the biogeographic provinces, Mediterranean, Temperate Forest and Patagonia. One of these species is Oligoryzomys longicaudatus (Long-tailed pygmy rice rat), which is parasitized by mites of the genus Mysolaelaps (Fonseca, 1936). In this context, the present study describes the parasitic infestation rates of Mysolaelaps microspinosus on O. longicaudatus and evaluates its relationship with biotic factors (sex and host body mass) and abiotic factors (biogeographical province and season). Of 328 O. longicaudatus analyzed, 68 Mysolaelaps positive cases were obtained. Prevalence and mean abundance were 20.7% and 0.708, respectively. Multifactorial logistic and negative binomial regressions were performed to evaluate the effect of biotic and abiotic factors on prevalence and mean abundance. Both rates were significantly and directly related with biogeographic province and season, with higher rates in Mediterranean and Temperate Forest provinces and in winter and spring. The mean abundance had a significant direct relationship with host body mass, describing higher rates on heavier individuals. These results suggest that environmental factors such as temperature and food availability, strongly affect the ecology of M. microspinosus. Here we proposed that the southern limit for the genus in Chile is 47°S, mainly, due to colder temperatures. Also, biotic factors such as the weight of unpregnant hosts are relevant in parasite infestations, due to a greater parasitic load capacity by heavier individuals.