The effect of precipitates on the tensile deformation (2 per cent strain) of a polycrystalline Cu-3.12Co alloy was studied. Deformation was inhomogeneous on an electron microscopical scale as well as on an optical microscopical scale. It was confined to widely separated slip planes or groups of planes. The surface slip topography was studied by light microscopy, 2-beam interferometry, 1-stage and 2-stage replication, and by transmission electron microscopy using 1-sided thinning, including a novel shadow-cast variation. Fine slip, with step heights down to 70 A and spacings down to 300 A, was observed by transmission electron microscopy. Neither the step height nor the step spacings were appreciably altered by the precipitation of coherent spherical cobalt particles with average diameters of up to 180 A. There was some evidence of the dissociation of dislocations and the pinning of the partials by particles. The work hardening rate was affected very little by the presence of particles.