The production of fibers is carried out by allowing a gas mixture containing a gaseous hydrocarbon or an appropriate gas to pass through a substrate [6] (generally a steel sheet) arranged facing the direction of the gas stream and situated in a furnace [4] wherein the gas reaches a temperature of 1 000 DEG C approximately. The schematized device is comprised of the corresponding gas reservoirs, that is to say the hydrocarbon gas [1] and the carrying and activating gas [2], a mixing and preheating chamber [3], the furnace [4] at the outlet [5] of which are collected or burnt the inflammable gases and the grid [6] from the surface of which, duly activated, the carbon fibers may grow. The gist of the invention is that the gas goes through the substrate situated facing the flow, so that said flow of gas is parallel to the direction of the fiber growth. The maximum length of the fibers is set by the distance between the substrate [6] and a substrate or mask (not shown in the figure) situated at the other extremity of the chamber.