Abstract: A sprinkler comprises a nozzle head connectable to a source of pressurized water and including a nozzle bore through which the water issues in the form of a jet, and a rotary deflector having an underface formed with a radial array of grooves each having axially-extending inlet ends, radially-extending outlet ends, curved junctures joining the two ends, and curved bottom faces, which grooves are impinged by the water jet and deflect the jet laterally of the sprinkler while rotating the deflector. An adjustable mounting is provided for the rotary deflector enabling it to be mounted with its rotary axis at a plurality of different positions with respect to the axis of the nozzle bore, to vary the water distribution pattern produced by the rotary deflector laterally of the spinkler.