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A 12-week-old sexually intact female domestic Dumbo rat (Rattus norvegicus) was presented for dystocia. The animal had maloccluded incisors, mucohemorrhagic vulvar discharge, and no evidence of obstruction of the pelvic canal. The animal was treated parenterally with calcium and oxytocin and provided supportive care. Controlled vaginal delivery under general anesthesia was unsuccessful. The animal was humanely euthanized and was diagnosed with bacterial placentitis on necropsy. Histopathologic examination revealed abundant filamentous, Gram-negative, Giemsa-positive bacteria associated with the placenta, endometrial surface, and uterine vessels of the dam, and alveolar spaces of multiple fetuses. Aerobic bacterial culture of placental tissues revealed small pure gray colonies of Gram-negative, rod-shaped, and filamentous bacteria. Polymerase chain reaction targeting the 16S ribosomal RNA gene followed by sequencing identified the etiologic agent as Streptobacillus moniliformis, the causative agent of rat bite fever and a zoonotic disease that historically has been considered a nonpathogenic commensal organism of the rat. The current report documents placentitis attributed to S. moniliformis infection in a rat and is consistent with reports in mice and humans that this organism may be an opportunistic pathogen of the female reproductive tract; it also suggests that S. moniliformis can cause primary disease in a rat.