Surface flow visualization on an axial-flow pump results in patterns of the skin-friction lines on various surfaces and these patterns provide important insight into the topology of the three-dimensional flowfield. In particular, critical points and lines within the patterns give characteristics of three-dimensi onal separation and the formation of vortical structures. Here, oil-paint tests show the topology for the formation of horseshoe vortices, the structure of three-dimensional corner separation, the initial development of a rotor tip leakage vortex, and the development of other important secondary flows. Surface flow visualization can also provide information concerning boundary-layer transition. In summary, the surface flow visualization experiment presented in this article allows one to give a fairly detailed description of the complex flow within an axial-flow pump.