An arrangement for regulating the combustion air mixture in the case of a gas engine operating on a lean fuel, and in the induction line of which there is an air-gas mixer, a lambda regulator being provided and following which there is, for varying the combustion air proportions, a positioning device which adjusts the air-gas mixer, and also an output measuring device for ascertaining the engine output. A manometer determines the actual value of the mixture pressure upstream of the inlet valves of the engine, linked with the value for the combustion air proportions. By adjusting the combustion air proportions, it is possible to regulate the mixture pressure to the output dependent desired value for the mixture pressure as ascertained in a device within the lambda regulator.