The three-dimensional structure of incompressible flow in a narrow, open rectangular cavity in a flat plate was investigated with a focus on the flow topology of the time-averaged flow. The ratio of cavity length (in the direction of the flow) to width to depth was l:w:d = 6:2:1. Experimental surface pressure data (in air) and particle image velocimetry data (in water) were obtained at low speed with free-stream Reynolds numbers of Re l = 3.4×10 5 in air and Re l = 4.3×10 4 in water. The experimental results show that the three-dimensional cavity flow is of the 'open' type, with an overall flow structure that bears some similarity to the structure observed in nominally twodimensional cavities, but with a high degree of three-dimensionality both in the flow near the walls and in the unsteady behaviour. The defining features of an open-type cavity flow include a shear layer that traverses the entire cavity opening ultimately impinging on the back surface of the cavity, and a large recirculation zone within the cavity itself. Other flow features that have been identified in the current study include two vortices at the back of the cavity, of which one is barely visible, a weak vortex at the front of the cavity, and a pair of counter-rotating streamwise vortices along the sides of the cavity near the cavity opening. These vortices are generally symmetric about the cavity centre-plane. However, the discovery of a single tornado vortex, located near the cavity centreline at the front of the cavity, indicated that the flow within the cavity is asymmetric. It is postulated that the observed asymmetry in the time-averaged flow field is due to the asymmetry in the instantaneous flow field, which switches between two extremes at large time scales.