Abstract: A method of making ducts having outwardly extending ribs formed integrally with the duct structure. Two tubes are formed from fiber reinforced resin materials, a uniform inner tube and a ribbed outer tube, then the inner tube is bonded with in the outer tube to form a high strength ribbed duct. An outer tube is formed by placing resin impregnated fibers over a cylindrical mandrel having an inflatable bladder on its surface, placing a plurality of spaced rings tightly against the outer surface, pressurizing the bladder to expand the tube into the spaces between rings to form low ribs, then gradually moving the rings toward each other to press the ribs therebetween and form thin upstanding ribs. The inner tube is formed by placing resin impregnated fibers on a slightly thinner mandrel that also has a surface bladder. When both tubes are formed, the mandrel is removed from the outer tube and the inner tube and mandrel are inserted therein and the inner tube bladder is inflated.