ABSTRACT: A 7.5 kg common carp Cyprinus carpio presented with prominent localized swelling in the caudal right coelomic area, identified by ultrasound as a fluid filled mass. Fine needle aspirate (FNA) and culture results suggested a sterile seroma. Centesis removed 290 ml of serosanguinous fluid that returned within days. Recheck ultrasonography revealed a solid component within the cavity. Radiography demonstrated irregular lysis and misalignment of vertebrae adjacent to the mass, most suggestive of bacterial osteitis or neoplasia. Treatment with antibiotics followed for 2 mo but failed to resolve the lesion. Repeated radiography and ultrasonography showed progressive enlargement of the mass, with vertebral lysis and invasion characteristic of neoplasia. Ultrasound-guided FNA of the solid component of the mass was non-diagnostic. Euthanasia was elected due to the poor clinical response and primary differential of neoplasia. Post-mortem MRI and CT confirmed a retroperitoneal soft tissue mass, partially surrounded by a fluid-filled cavity, causing vertebral lysis and infiltration of the spinal canal. Expansion of the mass caused severe muscle loss and an associated elevation in creatine kinase (>120000 U l^-1). Necropsy results corroborated the MRI and CT findings, revealing a retroperitoneal, multilobular, red and tan mass causing dorsal displacement of the vertebral column, with vertebral lysis, pathologic fracture and invasion of the spinal canal. Histopathologic examination revealed a locally aggressive neoplasm exhibiting multiple patterns of growth, including endothelial lined vascular channels and solid areas formed by more pleomorphic polygonal and spindle cells, consistent with hemangiosarcoma.