A fluid mixer including an inner fluid flow (11) duct having a cylindrical wall (14) provided with the window openings (13) and an outer tubular sleeve (12) disposed outside and extending along the duct (11) to cover the openings (13). Fluids to be mixed are admitted to one end of duct (11) through an inlet (25) and the mixture flows out through outlet (32). Duct (11) is statically mounted in pedestals (15) fixed to a base platform (17). Sleeve (12) is mounted for rotation in further pedestals (16) and driven by motor (23) and drive belt (22) to rotate concentrically about duct (11) such that parts of the sleeve move across the window openings (13) to create viscous drag on fluid flowing through the duct and transverse flows of fluid in the regions of the openings to promote mixing.