Abstract: A reaction arm for rotary sprinkler is made of injection molded nylon. The arm is formed in two parts including a head and a main body portion. The head contains the spray diverting and deflecting passageways and the main body portion has a head-supporting shoe on one end, an intermediate pivot portion, and a counterweight portion. The shoe is received within an opening in the head and forms one wall of the water passageway. A counterweight may be added to the arm adjacent the head to provide additional mass to the reaction arm for proper operation of the sprinkler.