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The diffusion coefficients of poly(beta-benzyl L-aspartate)(PBLA) with the alpha-helix (rodlike) form in chloroform and the random-coil form in a mixture of chloroform and trifluoroacetic acid have been measured as a function of the PBLA concentration (C(PBLA)) by using high field-gradient 1H NMR. The PBLA concentration range for the former is from 0.11 to 9.20 wt % and for the latter it is from 0.20 to 25.2 wt %. From these experimental results, it is found that the diffusion coefficient of PBLA in the rodlike form is much smaller than that of PBLA in the random-coil form at the same PBLA concentration. This implies that the small diffusion coefficients of PBLA in the rodlike form as compared with those in the random-coil form come from the relatively large radius of gyration Rg. Further, it is found that the diffusional behavior of PBLA in the rodlike form is different in the three PBLA concentration regions, that is, region 1 (C(PBLA) = 0.11 approximately 0.39 wt %), region 2 (C(PBLA) = 0.39 approximately 0.86 wt %), and region 3 (C(PBLA) = 0.86 approximately 9.20 wt %). In the random-coil form, the diffusion is different in the two PBLA concentration regions, that is, region I (C(PBLA) = 0.20 approximately 1.03 wt %) and region II (C(PBLA) = 1.03 approximately 25.2 wt %). These diffusional behaviors in the rodlike form and in the random-coil form can be reasonably explained by Tinland-Maret-Rinaudo theory and de Gennes theory, respectively. Further, transitional change from the rodlike form to the random-coil form is discussed on the basis of the diffusional behavior.