The aim of the study was to describe the diagnostic procedure, indications for operation and operative result in a patient with primary malignant melanoma of the rectum. In the patient, hospitalized at the Department of Surgery, School of Medicine in Zagreb, a malignant melanoma of the rectum was diagnosed by rectoscopy and histology. Tumor staging was determined by endorectal ultrasonography. Operation was indicated for malignant disease with partial obstruction of the rectum. Since the tumor was locally inoperable, with liver metastases a palliative operative procedure (sigmoidal colostoma) was performed. The operative finding confirmed the tumor stage obtained by ultrasonography. The patient died 5 months after the operation due to widespread dissemination of the malignant disease. Thereby, the accuracy of preoperative tumor staging by ultrasonography was confirmed. We believe that, in view of its accuracy, endorectal ultrasonography can be used for tumor staging in patients with primary melanoma of the rectum. Therefore, expensive diagnostic imaging methods like CT and MRI are no longer necessary.