Large-scale coherent structures in a large, single-stream plane mixing layer of air have been investigated experimentally. The unforced, initially fully turbulent mixing layer rolls up into organized structures whose average passage frequency fm at any downstream distance x from the lip depends on x. These structures are detected for the entire length of the measurement, i.e. up to x = 3 m or 5000θe. The Strouhal number Stθ (= fm θ/Ue) is observed to be a constant (≈ 0.024) at all x. θe and θ are, respectively, the exit and local momentum thicknesses of the mixing layer, and Ue is the free-stream velocity. (The entrainment velocity on the zero-speed side is found to be 0.032 Ue.) The coherent-structure properties are educed in the developing and self-preserving regions of the mixing layer using an optimized conditional-sampling method, triggered on the peaks of a local reference ũ-signal obtained from the high-speed edge of the layer. Sectional-plane contours of the properties of the structure such as coherent vorticity, Reynolds stress and production reveal that the structure formation and evolution are complete by x ≅ 500θe, beyond which the structure achieves an ‘equilibrium’ state as defined by the structure properties.