The invention is directed to a turbine (3) for generating power in a drill string (2), including a turbine rotor (10) drivable by a fluid and a bypass device having a first bypass channel bypassing the turbine rotor (10). For varying the volumetric flow through the bypass channel, provision is made for a valve (28) and a control device driven by the fluid and having a drive element (24) by means of which the valve (28) is movable anywhere between a first and a second position. The drive element (24) produces a flow resistance in the fluid flow path downstream from the turbine rotor (10) and downstream from the valve (28) and in the flow direction takes support upon a spring (26).