The full range of phenomena occurring in the flow of suspensions remains still unexplored. This paper describes phenomena observed in a neutrally buoyant suspension consisting of a single layer (a monolayer) of rigid spherical particles, flowing through a 220‐cm long rectangular channel of high‐aspect ratio (1:16) with its minor dimension(116in.)1.6 times larger than the particle diameter. The flow was investigated over a range of particle Reynolds numbers between 101 and 407, and of concentrations between 1.7 and 5%. Concentration distributions obtained from a detailed analysis of direct photographic observations show the existence of four consecutive regions as the suspension moves down the channel: (a) an initial region, close to the channel entrance, characterized by essentially uniform particle distribution. (b) A region in which the particles move away from the wall forming a particle‐free layer next to it. The core containing the particles does not have a uniform concentration distribution but exhibits marked concentration peaks at its outer margin. (c) A region in which the core has narrowed, the peaks have merged, and the concentration has become uniform. (d) A region of instability characterized by the onset of waves in the configuration of the core, followed, as the flow progresses downstream, by a breaking up of the core and by the formation of groups of particles separated by axial gaps of suspending liquid. The transitions between these regions are not sharply bounded, each region evolving gradually into the next. The process is accelerated by increases in flow velocity, and decelerated by increases in concentration. The phenomena point to the existence of a hydrodynamic force leading to the formation of the core, and possibly of concentration‐dependent particle interactions opposing such a formation and contributing to the onset of instabilities.