Transmission electron microscopy and microbeam x‐ray techniques were used to study the substructure developed in pure, polycrystalline aluminum during constant strain fatigue. A complete study was made at strain amplitude ±0.002 over the cycle range 0–100 000 cycles. During the first several thousand cycles, the substructure varies considerably from grain to grain, depending on orientation. In general, dislocations initially cluster along {111} glide planes during the first few hundred cycles. By 2000–3000 cycles subgrain boundaries have formed along {111}, {100}, and {110} planes. Large numbers of dislocation loops are seen in and near the boundaries. As cycling proceeds to fracture, the dislocation density in these boundaries and the misorientation between the subgrains increases. The average subgrain volume decreases from ∼25 &mgr;3at 5000 cycles to ∼8 &mgr;3at 100 000 cycles. Subgrains were formed at all strain levels which were investigated, the lowest being ±0.0005; however, the rate of formation decreases rapidly as the strain is lowered. A model of fatigue hardening is proposed based on these observations, and the relation to fatigue fracture is discussed briefly.