Contact with rat saliva or faeces can lead to infection with Streptobacillus moniliformis and the condition known as 'rat bite fever'. We report a case of septic arthritis of the hip due to this organism following a bite on the finger of a 14-year-old boy from a rat for sale in a pet shop. The case was successfully treated by arthrotomy, drainage and joint lavage followed by administration of penicillin. Septic arthritis of the hip due to S. moniliformis has not been previously described and this case highlights a possible danger of keeping rats as pets.