Introduction: ACTINOMYCETES are recognized anaerobic pathogens of man. The species usually causing disease are Actinomyces israelii and Actinomyces bovis. There are, however, actinomycetes that are facultative anaerobes; A. viscosus is one such organism, and in spite of being a commensal in the majority of human adults with teeth, it has rarely been reported to cause disease. We present a rare case of disseminated actinomycosis caused by Actinomyces viscosus in a patient on methotrexate. Case presentation: A 74-year male, presented to the emergency room with complaints about generalized weakness and multiple falls. The patient didn’t have any obvious respiratory complaints like a cough, shortness of breath or hemoptysis. His medical history was significant for psoriatic arthritis on methotrexate therapy for two years, hypertension and a recent admission for a skin infection and was being treated with amoxicillin for two weeks. The patient was an active cigarette smoker with significant smoking history. In view of the multiple falls, a trauma code was called in ER, and he underwent a pan CT scan as part of trauma evaluation which revealed no fractures but the presence of left upper lobe lung mass along with multiple pulmonary nodules and numerous lesions in the thoracic spine, peritoneum, and brain consistent with lung cancer with a metastatic process. The medical records from the previous hospital were obtained which revealed that the skin infection on the neck was cervical actinomycosis with biopsy positive for Actinomyces Viscosus infection and was discharged on Amoxicillin. During that admission, a Lung mass was also found for which an outpatient biopsy was planned. With this background, a possibility of disseminated infection vs. metastatic process was considered. Various subspecialists were involved, and risks and benefits were discussed concerning biopsy versus aggressive antibiotic therapy. Given the fact that patient was already on amoxicillin for two weeks, his medical records were obtained to compare the imaging, which included a previous CT scan of the chest. A detailed radiological comparison showed a significant reduction in the size of the lung mass seen during the last CT scan. Given this finding, a consensus was established that the multiple lesions found on imaging including brain lesions were secondary to a disseminated infection but not metastatic disease. Antibiotic regimen was then changed to IV penicillin G (describe the whole dosage regimen) for disseminated disease and methotrexate was held. A follow-up imaging in subsequent visits showed complete resolution of lung mass within four weeks and significant improvement in all the other lesions. Subsequent follow up showed full resolution of all other lesions with an overall six months of antibiotic therapy. Discussion: Actinomyces viscosus is usually known to cause disease in canines and other animals. Only a few cases have been reported in humans where it was mostly associated cervicofacial, pulmonary, perioperative endophthalmitis, endocarditis brachial cyst infections but never a disseminated infection. This is the first case we have seen in existing literature where Actinomyces Viscosus is involved in disseminated disease. Disseminated actinomyces infection has usually been seen with Israeli, Meyeri, odontolyticus History of methotrexate use could be a possible reason for the dissemination, as methotrexate use is associated with immunosuppression and complicated infections. Our case not only highlights an unusual presentation of rare disease but also signifies the importance of multidisciplinary involvement and rational thinking in the management of complicated cases like this. Physicians should be well versed with various species and presentations of Actinomyces infection. There is no clinical characteristic of A. viscosus infection that can distinguish it from other Actinomyces species. However, microbiologically A. viscosus is a not acid-fast, facultative anaerobe that grows as acute-angle branching filamentous rods. Unlike other species, A. viscosus colonies are catalase-positive and indole-negative.2 The diagnosis is frequently made by histopathological examination of excised tissue, which has a characteristic “sulfur granule” appearance grossly and on Gram stain contains branching gram-positive rods that grow only anaerobically.”Sulfur granules” contain filamentous which are Gram-positive but negative with acid-fast staining. This feature differentiates actinomycosis from nocardiosis. Actinomyces viscosus is different from the other Actinomyces as it is a being catalase positive organism Prolonged antibiotic therapy for 6 to 12 months, usually with penicillin or amoxicillin, is required since it has poor drug penetration into fibrotic lesions. In penicillin-allergic patients, tetracyclines are an alternative option. Adjunct therapy includes Surgical therapy involving drainage or excision. In our patient, the antibiotic treatment turned out to be effective, although he was immunocompromised and the lesions were disseminated. Actinomycosis usually shows a good response to antibiotic therapy; however, careful treatment and follow-up are required, especially in immunocompromised patients.