PURPOSE: To identify the characteristic appearances of masticator space infection, as seen on magneticresonance(MR) imaging. MATERIALS AND METHODS: We retrospectively reviewed the MR images of 23 patients withclinically and bacteriologically proven masticator space infection, with attention to the involved structures,spread pattern, abscess formation, mandibular involvement, and etiology. RESULTS: The masseter muscle wasinvolved in 21 of 23 cases, while the cases, medial pterygoid, lateral pterygoid, and temporalis muscles wereinvolved in 14, 12, and 13 cases, respectively. All muscles in the masticator space were involved in eight casesand only a single muscle in five. In all but one case, extension through the muscle plane was noted, and in 10cases, transfascial extension was seen. Abscess formation was noted in seven cases. Mandibular involvement wasseen in 16 cases, half of which showed focal cortical disruption. The source of infection was odontogenous in 15cases, with frequent involvement of the mandible and masseter. CONCLUSION: Masticator space infection frequentlyoriginated from an odontogenous source. The characteristic MR appearances of this infection included extensionthrough the muscle plane and frequent transfascial spread to adjacent deep cervical spaces, as well as commonmandibular involvement with or without cortical disruption of focal pattern.