放射菌本身為人體之固有菌種，廣泛存在人體的口腔、消化道、呼吸道內。放射菌的致病力不強，感染時常會同時合併其它細菌(mixed)感染，臨床上多以一般紅腫熱痛等發炎的症狀做為表現，但若時間夠長，在皮膚表面可生成多發性管，釋出帶硫磺顆粒的膿液。一般多發生在免疫低下的病人身上。在頭頸顏面部的放射菌感染並不少見，但單純在腮腺發生感染則為一罕見之情形，在臨床及影像學上，有時這樣的感染會有類似惡性腫瘤的表現，甚至是在感染的情況下同時潛藏腮腺腫瘤。本例為24 歲健康男性，在左側腮腺發生放射菌感染的情形，其臨床及影像學上的表現，與腮腺腫瘤有些許雷同之處，故特此討論。

Actinomycosis infection over cervico-facial area was common. However, primary actinomycosis of parotid glandwas a rare condition and sometimes couldmimicmalignancy under physical examination and image studies. A 24-year-old male patient visited our department with a complaint of progressing left parotid tumor for several months. Computed tomography revealed a heterogenous mass with adjacent soft tissue invasion. Under the impression of parotid tumor with secondary infection, the patient received incisional biopsy and drainage. The final pathologic diagnosis is actinomycotic infectionwith observed clusters of filamentous bacteria. The symptoms subsided immediately after the operation and oral antibiotic was subscribed for another 4 weeks. Complete clinical response was observed after 3 months of regular follow up.