The characteristics of seven strains of Streptobacillus moniliformis, including four isolates from a recent outbreak of Haverhill fever, are reported. Acid production from carbohydrates was uniform apart from variable reactions with mannose and salicin. Enzymatic reactions determined by the API ZYM system and fatty-acid profiles were generally consistent and may be of value in the rapid identification of S. moniliformis. Penicillin was the most active of the antibiotics tested in vitro, which supports its use as the drug of choice in the treatment of Haverhill fever.