The ultrastructure of the curly fibers was examined by the transmission and immunoelectron microscopy as well as by the rapid-freeze, deep-etch and replica method. The curly fibers consisted mainly of paired helical filaments (PHF) in the neuropils, both pre- and post-synaptic. On the deep-etch replicas, PHF in the neurites had similar dimensions to those of Alzheimer's neurofibrillary tangles in the nerve cell somata, having the width of 28 to 36 nm. The two component filaments, 14 to 18 nm in width, were twisted left-handedly with about 70- to 90-nm intervals. There were also cross-bridging fibrils of about 6 nm in diameter connecting the neighboring PHF.