A once-through steam generator includes a combustion chamber, the walls of which comprise vertically arranged evaporator pipes connected to one another in gas-tight fashion by pipe webs, through which evaporator pipes flows a flow medium from bottom to top. The evaporator pipes are combined by upstream inlet collectors to form more intensely and less intensely heated pipe groups. A feed water supply is assigned to respective inlet collectors. At least one regulating valve regulates throttling of the mass flow of the flow medium into the evaporator pipes. To determine a control variable for the regulating valve, temperature measurement device measures outlet temperatures of the flow medium exiting the evaporator pipes. Each of the more intensely and less intensely heated pipe groups is assigned to one of the inlet collectors and to an outlet collector, and each of the outlet collectors has one of the temperature measurement devices.