A six-month-old, 3.3-kg, intact male American shorthair cat with a ventricular septal defect (VSD) underwent open-heart surgery using total cardiopulmonary bypass. Repair of the VSD was performed via a right fifth intercostal thoracotomy using a right atrial approach. To avoid excessive blood dilution, blood cardioplegia, retrograde autologous priming, conventional ultrafiltration, and modified ultrafiltration were used. After surgery, the shunted flow through the VSD resolved. The cat was alive without medical therapy or clinical signs one year after surgery. Information on open-heart surgery using cardiopulmonary bypass in cats, particularly with regard to hemodilution and restrictions on transfusion volume, is limited. Various measures may be useful for safe achievement of complete extracorporeal circulation with aortic clamping in cats, given the limitations of feline blood transfusion.