In the representative embodiments of the present invention described herein, a new and improved directional drilling tool carrying a rotable drilling bit is dependently coupled to a rotatable drill string for excavating a borehole along one or more selected courses of excavation. Passage means on the directional drilling tool are communicated with the several fluid-discharge passages in the drill bit. A rotating flow obstructing member is arranged in the directional drilling tool for selectively communicating the drilling fluid with the bit passages as the drill bit is rotated. A selectively operable driver is arranged for rotating the flow-obstructing member at one rotational speed when it is desired to sequentially discharge drilling fluid into most, if not all, adjacent sectors of the borehole as the drill bit rotates so as to advance the drill bit along a generally linear course of excavation. Alternatively, by rotating the flow-obstructing member at a different rotational speed, drilling fluid is sequentially discharged from the bit passages into only a single peripheral sector of the borehole to divert the drill bit to another course of excavation. Various controls are disclosed for utilizing typical directional measurements to control the direction of excavation.