The light and electron microscopic characteristics of hyaline deposits present in the pancreatic islets of two aged diabetic cats are described. The hyaline deposits were consistent with amyloid when sections stained with periodic acid-Schiff (PAS), crystal violet, Puchtler's Congo red, and Masson's trichrome were examined by bright field illumination. Green birefringence with Congo red-stained sections and red birefringence with toluidine blue-stained sections were apparent with polarized illumination. In addition, fibrillar components typical of amyloid were ultrastructurally demonstrated in these deposits. Hypotheses regarding the cause of localized amyloidosis in pancreatic islets are considered. Future investigation of the pathogenesis of amyloid deposition in feline pancreatic islets is warranted to obtain a better understanding of the pathogenesis of maturity-onset diabetes in man and the cat.