Three adult koi fishes were submitted to the veterinarian for the physical examination due to the presence of abdominal distension and swimming difficulty. All fishes presented enlarged abdomens which asymmetrical lateral protrusion to the one side of the body. The abdomens were firmly palpated, and no pain response was observed. Surgical treatments were taken to expose and extract any foreign abdominal content. A special surgical table and water run-off unit were set to support the fishes’ homeostasis during surgery.  The fishes were sedated using combination of phenoxyethanol (0.5 ml per litre) and eugenol (0.1 ml per litre) dissolved in water and administrated through gills. The fishes were then positioned in dorsal recumbency underlaid by wet cloth on the hanging plastic drapes. A paramedian approach coeliotomy was performed by making 10-12 centimetres incision through skin aside the midline. Removal of any coelomic foreign body was done by gently palpating and pushing out the space-occupying fluid or masses. The contents were then extracted and preserved in 10% non-buffered formalin. Polyglycolic acid 3-0 and polypropylene suture threads were used to close the internal tissue and skin respectively. The foreign abdominal contents consisted of multiple whitish solid masses and several tightly attached cysts containing yellowish fluid. The underlying tissues were not visually recognized. There were multifocal to coalescing, moderately demarcated, round, firm nodules in various size ranging from one-to-five centimetres in diameter. The larger coalescing masses were measured up to ten centimetres at their longest axis. Histopathologic examination of the masses revealed stromal and core area consisting of spindle-shape and atypical cells respectively. Higher magnification showed fibrovascular structure infiltrated by mononuclear cells in the cortex. Various size of polygonal and atypical cells occupied in the core of the masses. These descriptions led us to identify the masses as sex-cord stromal tumours.