In accord with control valve 1 of the present disclosure, an operating valve member, as generally indicated at 29, is mounted within chamber 17 for movement in response to changes of temperature of the operating fluid contained within the chamber between a closed position (as shown in FIG. 1) and a fully open position. The valve member 29 is preferably in the shape of an elongate rectangular strip having side edges 29a, 29b, a proximal end 29c which is preferably fixedly cantilever supported with respect to end wall 25a of housing 15, and a free distal end 29d. However, within the broader aspects of this disclosure, valve members of other shapes may be used. When valve member 29 is in its closed position, the distal end 29d of the valve member is above orifice 27 such that the valve member substantially blocks the flow of fluid through chamber 17 to orifice 27. When the valve member 29 is in its fully open position (not shown in FIG. 1), the distal end 29d of the valve member is below orifice 27 such that the flow of fluid from chamber 17 to outlet 11 through the orifice is substantially unimpeded by the valve member. As noted, housing bottom wall 21 is preferably curved such that it matches the curvature of the valve member 29 as the latter moves or flexes between its closed and open positions, and as the valve member flexes above its closed position. With the valve member 29 in its fully open position, the distal end 29d of the valve member is positioned below the bottom of orifice 27 so that it does not interfere with fluid flowing through the orifice.