The self-diffusion coefficient, D-s, of fluorescently tagged poly(gamma-benzyl alpha,L-glutamate) has been measured throughout the isotropic regime in pyridine. Intrinsic viscosity, phase boundary studies, epifluorescence microscopy, computer modeling, and the diffusion rates themselves show that the labeling does not appreciably perturb the semiflexible rod structure of the polymer. It is also demonstrated that pyridine is a good solvent for poly(gamma-benzyl alpha,L-glutamate), comparable to N’,N’-dimethylformamide. As concentration increases, two or three regimes are found depending on the rod length. Dilute behavior holds to number densities, nu, well exceeding L(-3), where L is the rod length. A somewhat better estimate of the point at which D-s begins to decrease is the classical criterion, c approximate to [eta](-1) where c is the weight/volume concentration and [eta] is the intrinsic viscosity. Over the whole range of molecular weights, the condition nu dL(2) approximate to 0.5-1, where d is the rod diameter, well describes the downturn number density. This corresponds to nu/nu(*) approximate to 0.1-0.2, where nu(*) = 16/(pi dL(2)) is the Onsager critical number density associated with the lyotropic liquid crystal transition. Once diffusion begins to decrease, it does so strongly, obeying D-s similar to C(1.13+/-0.04)L-(1.8+/-0.2). The third regime of diffusion, evident only in the two longest samples, is again relatively level and lasts to the lyotropic transition. Diffusion does not cease at any concentration, and D-s is reasonably well scaled when : number density is normalized by nu(*), which underscores the importance of finite thickness effects for semidilute solutions of poly(gamma-benzyl alpha,L-glutamate). The self-friction and mutual friction coefficients differ, with the former being lower in dilute solutions.