Morphologic alterations in cortical fiber cell membranes of the developing Emory mouse cataract were studied with scanning, transmission and freeze-fracture electon microscopy. Scanning electron microscopy revealed the extensive formation of prominent ridges on the surfaces of normal-appearing fibers, greatly enlarged degenerating fibers and globular structures in the relatively superficial cortical regions of the cataractous lenses where such a surface pattern was not found in the normal controls. Transmission electron microscopy showed undulating 13 nm pentalamellar structures, which were thinner than 17 nm heptalamellar (or pentalamellar) structures of gap junctions, were distributed within the cell membranes having ridge patterns. Some globular structures were encircled by repeated undulating 13 nm pentalamellar structures and multi lamellar membranes. Freeze-fracture studies demonstrated that 13 nm pentalamellar structures consisted of square crystalline arrays of 6 nm intramembrane particles whereas 17 nm heptalamellar profiles showed randomly-packed 9 nm intramembrane particles of typical lens fiber gap junctions. It is suggested that the extensive formation of ridges in the relatively superficial cortical regions of the Emory mouse lenses may be associated with a degenerative process of lens fiber cell membranes during cataractogenesis.