The diameter and conduction velocity of myelinated fibers in the sciatic nerve isolated from neurofilament-deficient quail ('quiver') were compared with those from wild quail. The diameter was significantly reduced in quiver, however, the frequency distribution formed three peaks as did that observed in the wild. The fibers were categorized in three groups possessing different conduction velocity. The conduction velocity appeared to be proportional to the diameter in each strain, and was significantly reduced in quiver. These results indicate that, although the diameter and conduction velocity are reduced in quiver fiber, the correlation between the parameters observed in wild quail is still preserved.