Abstract: A diffuser for aerating a fluid, and particularly for aeration in waste water treatment tanks, is formed in an elongate configuration by manufacturing a base plate from an extrusion cut to the desired length. A perforated elastomeric membrane is secured to the base plate by pressing sealing strips along the elongate edges of the membrane into corresponding grooves formed in the base plate extrusion. The ends of the membrane are sealed to the base plate by end strips or clamps. Air is introduced through apertures in the base plate causing the membrane to inflate away from the base plate forming an interior compartment for the passage of air along the diffuser. Apparatus for securing the diffuser to a horizontal header pipe is also described.