Three-dimensional dynamic stall was observed on the rotor of a fully equipped subscale helicopter model in a wind tunnel. Results of stereoscopic particle image velocimetry measurements on the retreating blade clearly revealed the large-scale dynamic stall vortex at 50% and 60% blade radius. The small-scale shear layer vortices that constitute the large-scale dynamic stall vortex were identified and localised. Based on the spatial distribution of these small-scale vortices, the characteristic large-scale structure was found to be unexpectedly compact. Along with an apparent spanwise flexion of the dynamic stall vortex, these results suggested the rotational motion of the rotor to have a stabilising effect on the formation and convection of the dynamic stall vortex. The velocity field information available provides a basis for validating recent computational fluid dynamics results of three-dimensional dynamic stall. © 2011 Elsevier Masson SAS. All rights reserved.