We present the clinical observation of a female patient with cystic peritoneal malignant mesothelioma developed in the thickness of the abdominal wall. The diagnosis included several steps: tumor classification as mesothelioma, tumor differentiation from reactive mesothelial hyperplasia, establishment of the malignant nature and differentiation from other malignant peritoneal tumors. Relapse in about one year after surgery and about six months after the end of chemotherapy also claim malignancy of the tumor. The particular tumor location in the thickness of the abdominal muscles, seemingly without involvement of the parietal peritoneum, in a patient with a history of caesarean operation, questions its development out of ectopic tissue embedded in scar from previous surgery.