Abstract: Employee #1 had been contracted to remove a tail bearing housing on a National 456 oil pumping unit, type D325b-456DW. He stopped the unit, secured the walking beam with chains, then climbed up to the walking beam and walked down to the tail bearing housing. He had removed the U bolts and nuts when the walking beam rotated. The rear position swung downward, contacting an upright Sampson post, which caused the four saddle bolts at the pivot point to fail. Employee #1 fell to the base of the unit and was struck by the falling walking beam and killed. Causal factors include not securing the down chain on the front of the walking beam in order to prevent it from sliding down the beam.