Streptobacillus moniliformis is the causative agent of rat bite fever, with endocarditis being a rare but well-documented complication. We report the case of an HIV-positive man who acquired S. moniliformis endocarditis through a rat bite. No predisposing cardiac lesion was known. He was treated with ceftriaxone 2 g/day i.v. for 3 weeks, gentamicin 120 mg/day i.v. for 2 weeks and penicillin 24x10(6) units/day for 1 week. At the end of the antibiotic therapy he suffered a generalized Candida albicans infection, which was treated with fluconazole for 1 week. He was subsequently discharged in a satisfactory condition.