The evidence of the existence of natural foci of HFRS in Penza region has first been presented on the basis of serological examinations of patients and normal population as well as HFRS virus antigen demonstration in the lung tissue of small mammals. An outbreak of HFRS among children in a pioneer camp is described. Aerogenic infection of the children contracted from HFRS virus-infected bank voles (Clethrionomys glareolus) living near the camp is assumed. The detection of HFRS antigen in 5 species of small mammals indicates the polyhostal nature of HFRS foci existing in the region.