In a turbine for an exhaust gas turbocharger having a turbine housing with a turbine rotor rotatable supported therein and including spiral channels for directing exhaust gas onto the turbine wheel, at least one annular blocking element is supported between the spiral channels and the turbine wheel so as to be rotatable in the peripheral direction of the turbine wheel and additionally movable in the axial direction of the turbine wheel for a controlling the exhaust gas flow to the turbine wheel between impulse turbine mode when extended into the space between the turbine wheel and the spiral chamber and an accumulation made when retracted, with the gas flow through the turbine wheel or by-passing the turbine wheel being adjustable by rotation of the blocking element.