A commutating valve for a downhole steering tool. The downhole steering tool has a sleeve which surrounds an annular body which in turn surrounds a driveshaft. A plurality of steering cylinders are located in the annular body and a steering piston is located in each steering cylinder, the steering pistons engaging the sleeve. A pump pressurizes hydraulic fluid within a reservoir, the reservoir having a closable outlet whereby the pressure of the hydraulic fluid within the reservoir can be raised by closing the outlet and lowered by opening the outlet. The valve comprises a number of discrete valve members which are all in communication with the reservoir and can be opened sequentially whereby to deliver hydraulic fluid to the steering cylinders sequentially. The pressure within each steering cylinder can be controlled in order to control the position of the driveshaft within the sleeve.