Actinomyces spp. cause a chronic suppurative, gramilomatous disease which is characterized clinically by extensive abscess formation, recurrent draining of sinuses and fistulae and histologically by the presence of the so-called "sulphur granules". Colonic actinomycosis is a relatively rare infection and its diagnosis is difficult. We report the case of a female patient who was operated on for ovarian cyst and the case of another female patient operated on for a mass in the transverse colon. In both cases the pathology of the excised tissues revealed actinomycosis. Actinomycosis must be considered in the differential diagnosis of patients who present with abdominal pain, fever, leucocytosis and intestinal wall thickness and/or abdominal mass.