The paper reports on a transmission electron microscopy analysis of the dislocation structure of normalized 09G2S steel with strain aging. The analysis shows that in a steel specimen deformed to its yield stress, the dislocation density peaks at the center of a strain localization band (in its active zone) and decreases by about an order of magnitude at its sides. The structure of these band zones, which are in different strain hardening stages, is examined and the Burgers vector of dislocations of the primary slip system is determined. The ends of such dislocations form ordered planar dipole walls, leading to polygonization by dislocation slip. Also considered is the specimen structure at the yield and ultimate stresses.