A 74-year-old woman presented with erythema of the extremities, a high fever and arthralgia after being bitten by a rat. The patient was diagnosed as having rat-bite fever based on the symptoms and clinical course, as well as the polymerase chain reaction detection of Streptobacillus moniliformis DNA in the crust of the bite site. This is the first case to be diagnosed using polymerase chain reaction on a crusted skin lesion specimen. Although clinical symptoms initially remitted with minocycline therapy, they relapsed. Subsequent administration of piperacillin sodium resulted in complete disappearance of the high fever and arthralgia.