A 9-month-old male Japanese domestic cat showed pleural effusion, ascites, azotemia, hypoproteinemia and severe proteinuria. Histopathology of the percutaneous renal biopsy specimen revealed that all glomeruli showed intense mesangial hypercellularity with an increased mesangial matrix and thickening of the capillary walls, resulting in lobular accentuation of the glomerular tufts. Frequent duplication of the capillary walls was also observed. Immunostaining for alpha-smooth muscle actin distinctly revealed mesangial interposition. Diffuse global and linear deposition of C3 and IgG was observed mostly along the peripheral capillary loops. Electron microscopy confirmed frequent circumferential mesangial interposition and subendothelial dense-deposits in the glomerulus. The glomerular lesion was consistent with human membranoproliferative glomerulonephritis type I, and might be a rare case that developed at young age.