Experiments have been performed in clear-plastic diffusers, using water flow discharging into a plenum tank, to obtain performance and flow-régime information for a wide range of conical diffuser geometries. A line of first appreciable stall has been located with sufficient precision that it is useful in conical diffuser design. Performance data obtained allow solution of the four common optimum problems encountered in diffuser design. At present these results can be extrapolated only with caution to diffuser geometries other than those tested.