Abstract: A preferred form of the invention contemplates a two-piece nozzle construction for use in assembled relation to the tube sheet of an underdrain filter system, wherein each of two nozzle halves is assembled to a tube-sheet opening, solely by access from the downstream side of the tube sheet, and with each of the two halves holding the other in the thus-assembled relationship; in the assembled relationship, the closed upstream end of the upstream half extends a substantial distance into lower bed material of the filter, with flow access to the nozzle via a distributed plurality of relatively small openings in the cylindrical body wall of the upstream half. The downstream half is open at both ends and, in assembled condition, extends downward from the tube sheet; this downstream half has side ports to assure uniform air flow up through the filter bed, in a backwash mode of the filter system.