Here, we report a patient developing a postoperative peritoneal infection by Aspergillus fumigatus. While galactomannan serum levels were negative throughout the time course, galactomannan levels in peritoneal fluids yielded high results. Serological testing of peritoneal fluids for fungal antigens might be a useful and easily applicable tool to support diagnosis of intraabdominal aspergillosis, which represents a rare type of invasive fungal infection.