A case of actinomycosis of the urachus is described. A 25-year-old woman presented with a midline mass below the umbilicus. The differential diagnosis included an inflammatory or a neoplastic lesion. Diagnosis was made by histologic examination, after complete excision of the tumour. Post-operative penicillin was given and the patient’s recovery was uneventful. A review of the literature is presented with special reference to the pathogenesis and treatment.