Rat bite fever (RBF) is rare in Denmark. It can be very difficult to diagnose, as the causative agent is an extremely fastidious organism which needs microaerophilic conditions to grow. We report a case of arthritis in the right ankle due to Streptobacillus moniliformis in a three-year-old girl who fully recovered after 14-day treatment of intravenous administrated penicillin followed by four weeks of orally administrated amoxicillin. Social history revealed the presence of two domestic rats living in the child's home. As rodents are becoming more popular as pets, RBF must be considered in patients with a history of relevant exposure.