Pressure-driven filtration by porous membranes is widely used in the production of drinking water from ground and surface water(1-3). Permeation theory predicts that filtration rate is proportional to the pressure difference across the filtration membrane and inversely proportional to the thickness of the membrane(4). However, these membranes need to be able to withstand high water fluxes and pressures, which means that the active separation layers in commercial filtration systems typically have a thickness of a few tens to several hundreds of nanometres(5). Filtration performance might be improved by the use of ultrathin porous silicon membranes(6) or carbon nanotubes immobilized in silicon nitride(7) or polymer films(8,9), but these structures are difficult to fabricate. Here, we report a new type of filtration membrane made of crosslinked proteins that are mechanically robust and contain channels with diameters of less than 2.2 nm. We find that a 60-nm-thick membrane can concentrate aqueous dyes from fluxes up to 9,000 l h(-1) m(-2) bar(-1), which is similar to 1,000 times higher than the fluxes that can be withstood by commercial filtration membranes with similar rejection properties(1,10,11). Based on these results and molecular dynamics simulations, we propose that protein-surrounded channels with effective lengths of less than 5.8 nm can separate dye molecules while allowing the ultrafast permeation of water at applied pressures of less than 1 bar.