Poly(γ-benzyl-L-glutamate) (PBLG) is a synthetic polypeptide with a rigid helical conformation in several solvents. Small-angle X-ray and neutron scattering measurements were used to study the reversible gelation process in semidilute isotropic solutions of PBLG in benzyl alcohol. Time-resolved measurements of the scattering signal due to aggregation of PBLG helices into a network of microfibrils show characteristics of a nucleation and growth process with an incubation period which is extremely sensitive to the gelation temperature. Analysis by the Avrami equation yields an exponent of about 2. A mechanism of homogeneous nucleation and one-dimensional growth of fibrils, which incidentally merge or branch to form the network, is proposed. The thermodynamic transition temperature controlling fibril nucleation seems to be below the gel melting temperature. This transition may be an incongruent melting point of an ordered crystalline phase, which lies within the wide biphasic region of the phase diagram.