Abstract: The burner for gaseous fuels is especially intended for boilers. It is provided with sliding and/or turning means (15, 37) so as to assure, at least, in the output range of between 50 and 100% of the rated output, an at least approximately constant quantity relationship between gas and combustion air. For this purpose, for the simultaneous change in the admission of gas and air into a mixing chamber (24), there is used a control component, for example, a piston (15, 37). A first moveable, turning and/or sliding control piston (15) is provided the outer cylindrical surface of which controls the amount of combustion air while inside the first, tubular control piston (15) there is a second control piston (37) preferably coaxial to the first control piston (15), which controls the amount of gas by means of its inner cylindrical surface. The two control pistons (15, 37) are firmly attached to one another and can be pushed back and forth and/or turned by means of an actuating device (33).