A regular pattern of parallel reflections with a periodicity of approximately 39 microns has been detected on peripheral nerve fascicles. The reflections have been found to originate from a layer of wavy epineurial collagen fibrils arranged in register around the entire circumference of the fascicle. The waves were observed to be disposed in parallel with the plane of the fascicular sheath and along its axis. The pattern was observed in cut or relaxed fascicles in situ as well as in isolated and split layers of the epineurium. The pattern was not observed on nerve fascicles under tension. An additional structural feature consisting of longitudinally disposed elastic fibres was also detected among the epineurial collagen fibrils. The perineurial collagen associated with sheets of cells was found to be disposed in flattened waves as in the inner layer of the epineurium. The period of the waves, however, was much shorter, in the range of 6-9 microns. From the nature of the wavy structure it can be surmised that on stretching or contraction the sheath length may change, thus accommodating displacement and movement of nerve fibres.