Diagnosis: Rat bite fever (RBF) due to Streptobacillus moniliformis. A Gram stain showed highly pleomorphic, gram-negative rods that were identified as S. moniliformis (figure l).The patient was given a regimen of penicillin (2 million U every 4 h intravenously). RBF has been associated with serious sequelae, such as endocarditis, hepatitis, meningitis, and nephritis. The patient did not meet Duke's criteria for diagnosis of endocarditis [1]. A transthoracic echocardiogram did not show vegetations. There was no evidence of involvement of other organs. Subsequent blood cultures showed no growth. The patient was treated with 7 days of intravenous penicillin, followed by 7 days of oral penicillin. She remained afebrile, no new skin eruptions appeared, and all residual lesions desquamated and healed. She did not return for her scheduled follow-up appointment. In a telephone interview that was conducted a week after she finished her course of penicillin, she stated that she had mild arthralgias but otherwise felt well, with no new skin eruptions. Six months after the initial presentation, the cryoglobulinemia had resolved, the antinuclear antibody titer had decreased to 1:40, and the patient remained symptom free. More than 2 million animal bites are reported each year in the United States, of which 1b% are attributable to rats [2]. t e ited States, of ic ~-1 are attributable to rats [2]. Although RBF is considered to be a rare illness, it is likely that it is underdiagnosed. S. moniliformis is found in the nasopharynx of most rats and is excreted in their urine. Nasopharyngeal carriage rates in healthy laboratory rats range from 10% to 100%; rates among wild rats have been estimated at 50%-100% [2]. The organism is usually transmitted by a bite or scratch from an infected animal, but it may also be acquired by simply handling the animal or from exposure to rat urine when handling cage materials [2]. On further questioning, the patient mentioned that she regularly cleaned the cages of all of her animals. She also informed us that she let her pet rats lick her teeth! RBF is characterized by the acute onset of shaking chills, fever, vomiting, and severe myalgias. After a few days, a maculopapular skin eruption appears, followed by arthralgias in up to 50% of cases. The eruption is usually seen on the extensor surface of the extremities and frequently involves the palms and soles. It can also be petechial, purpuric, or pustular and lasts for -3 weeks. Approximately 20% of the rashes undergo desquamation [3]. Usually the illness is self-limited. However, in a small number of untreated cases, serious sequelae, such as meningitis, endocarditis, hepatitis, nephritis, and brain or joint abscess may develop, with a mortality rate of 13% [4], increasing to 53% in cases with cardiac involvement [5]. Nineteen cases of S. moniliformis endocarditis have been reported; these cases occurred predominantly in patients with previously damaged heart valves [5, 6]. However, RBF is not a reportable disease, and the actual incidence of endocarditis associated with RBF is not known.