As part of studies on the nature of the endemic virus infections in natural rodent hosts, the possible association of cyst forms of Pneumocystis spp. with the presence of hanta-, cowpox-, and arenavirus antibodies in wild mice (Apodemus flavicollis, N=105; Apodemus agrarius, N=63; Micromys minutus, N=50) and the common shrew (Sorex araneus, N=101) was studied in south-central Finland. One hantavirus (Saaremaa virus, SAAV) seropositive A. agrarius, and 2 cowpoxvirus (CPXV) seropositive S. araneus were detected, and antibodies against an arenavirus (Lymphocytic choriomeningitis virus, LCMV) were found in all 3 mouse species but not in shrews. Cyst forms of Pneumocystis spp. were detected in all species except A. agrarius. There was no significant association between virus antibodies (LCMV in mice, and CPXV in shrews) and cyst forms of Pneumocystis in any of the species. Concurrent presence of virus antibodies (LCMV) and cyst forms of Pneumocystis were detected only in 1 M. minutus. In conclusion, we found no evidence of any association between Pneumocystis and antibodies to any of the viruses tested.