NOVELTY - Method for producing fermentation products from organic feedstock in a reactor (21), involves (a) producing enzymes for the enzymatic degradation of organic feedstock by an aerobic microorganism, (b) allowing enzymatic degradation of the organic feedstock to fermentable sugars, and (c) performing fermentation of the fermentable sugars to fermentation products by an anaerobic microorganism.    USE - The method and reactor are useful for producing fermentation products from organic feedstock containing polysaccharide including cellulose, hemicellulose or starch and lignocellulose (both claimed). The lignocellulose-containing feedstock is chosen from wood, straw, Miscanthus, switchgrass, and municipal solid waste. The fermented products are organic substances chosen from ketones (acetone, diethyl ketone, or methyl ethyl ketone), esters (methyl, ethyl, or propyl long-chain alkanoates), alkanes (methane, ethane, propane, butane, or long-chain n-alkanes), and carboxylic acids (acetic, propionic, butyric or lactic acid), preferably alcohols (methanol, ethanol, propanol or butanol). The fermentation products are used for producing desired product e.g. biofuel.    ADVANTAGE - The method can be performed in batch as well as in continuous mode, involves less elaborated process, is cost-effective, requires no washing and detoxification processes as the microorganisms are resistant to toxic substances produced during the process and hence product recovery and purification can be performed simply, can be performed with high substrate concentration, and enables high yield of product due to higher cell density by natural immobilization and with less substrate consumption.    DETAILED DESCRIPTION - Method for producing fermentation products from organic feedstock in a reactor (21), involves (a) producing enzymes for the enzymatic degradation of organic feedstock by an aerobic microorganism in an aerobic zone, (b) allowing enzymatic degradation of the organic feedstock to fermentable sugars, and (c) performing fermentation of the fermentable sugars to fermentation products by an anaerobic microorganism. The reactor comprises an oxygen permeable membrane (28) for separating an oxygen supply compartment (30) from a fermentation compartment (31), where the oxygen permeable membrane contains a surface facing the fermentation compartment, and the above-mentioned processes take place in the fermentation compartment of the reactor. A biofilm (34) is placed on the surface and comprises at least one strain of an aerobic microorganism in the aerobic zone of the fermentation compartment for the production of enzymes and further comprises at least one strain of an anaerobic microorganism in the anaerobic zone of the fermentation compartment for the fermentation of fermentable sugars, where the aerobic microorganism is supplied with oxygen through the oxygen permeable membrane. An INDEPENDENT CLAIM is included for the reactor which comprises the above-mentioned units, where the oxygen supply compartment accommodates a fluid containing oxygen and the fermentation compartment accommodates a base material mixture (22) containing organic feedstock.    DESCRIPTION OF DRAWING(S) - The drawing shows schematic view of biofilm reactor.    Reactor (1)    Base material mixture (22)    Oxygen permeable membrane (28)    Oxygen supply compartment (30)    Fermentation compartment (31)    Biofilm (34)