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The low-frequency excess electrical noise has been measured on carbon fibers with a wide range of crystalline perfection and corresponding electrical and mechanical properties. Fibers include those prepared from ex-PAN and ex-pitch polymers, and a catalytic-chemical vapor deposited filament. The extensional (Young's) moduli of these fibers varied from about 220 to 890 GPa (35–130 Msi), while the electrical resistivities varied from about 19 to 1 µΩ m. The low-frequency electrical noise of each fiber was found to be proportional to l2 and to vary as 1/fα, where f is the frequency and α is about 1.15. The most striking feature of the results was the strong dependence of the normalized noise power on the degree of crystalline perfection. Journal of Applied Physics is copyrighted by The American Institute of Physics.