Actinomycosis is a chronic suppurative and granulomatous-disease caused by Actinomycosis israelli. Clinical presentation of the abdominal form of actinomycosis is nonspecific-pain, fever, leukocytosis, increased erythrocyte sedimentation rate, a sensation of abdominal mass and a formation of fistula. In addition, abdominal actinomycosis may mimic a carcinoma, diverticular abscess, inflammatory bowel disease, and tuberculosis. Most of abdominal actinomycosis develops after trauma, appendicitis, diverticulitis or gastrointestinal perforation. We report a case of abdominal actinomycosis preoperatively mimicking as colon carcinoma, which had no predisposing factors. (Korean J Gastrointest Endosc 2003;27:8487)