In-situ deformation experiments in the high-voltage electron microscope have been performed on the precipitation-hardened alloy Al-1 at.% Ag, isothermally aged at 413 K for different times, in order to explain a maximum of unknown origin in the critical resolved shear stress during ageing. The dislocations move in a jerky manner. In the stable positions, they bow out between precipitates. The spectrum of obstacle distances h (I) and the mean obstacle distance I along the dislocation line as well as the curvature of the dislocation segments have been evaluated from the electron micrographs. The obstacle distances are longer by a factor of about five than those determined from the size and density of the precipitates. The dislocation curvature corresponds to a stress which is higher than the macroscopic yield stress. Data in the literature suggest that there is a re-solution process between epsilon and eta particles in the alloy. The flow stress is assumed to be mainly controlled by the eta precipitates. Their high strength cannot solely be due to stacking-fault hardening. Elastic interactions between the particles and dislocations and/or order hardening may additionally contribute to the flow stress.