The self-diffusion coefficients (D) of rodlike poly(n-alkyl L-glutamate)s having n-dodecyl side chains in the thermotropic liquid crystalline state were measured as a function of the main-chain length [molecular weight (M-w) of 7000, 30 000, and 130 000, which correspond to the main-chain lengths (L) of ca. 30, 200, and 890 Angstrom, respectively)] within the temperature range from 50 to 80 degreesC by means of the pulse field-gradient spin-echo H-1 NUR method to elucidate the diffusional behavior of the polypeptides in the thermotropic liquid crystalline state. From the experimental results, it was found that at temperatures above the melting point of side-chain crystallites in poly(n-alkyl L-glutamate) the polypeptide forms the thermotropic liquid crystalline state, and then the isotropic diffusion coefficients (D-iso) of the rodlike polypeptides are decreased with an increase in the main-chain length. The diffusion process was analyzed by the Kirkwood theory of diffusion process for rodlike polymers. The diffusion coefficients of poly(L-glutamates) in the directions parallel (D-parallel to) and perpendicular (D-perpendicular to) to the alpha-helical axis were determined, and the D-parallel to value was found to be larger than the D-perpendicular to value.