Experiments arc described in which the occurrence of flow separation under hydraulic jumps is studied as a function of Froude number, Reynolds number and condition of inflow. It is found that separation occurs quite generally, that it extends beyond the conventional downstream end point of the hydraulic jump, and that its severity tends to increase with increasing Froude number and to decrease with increasing Reynolds number. In any case, however, the areal extent and thickness of the separation cavity is very much larger for jumps with undeveloped inflow than it is for their counterparts with fully developed inflow.