At about 1630 eastern time, a Mooney M20C struck a ground marshaller during taxi after landing at Lakeland Linder Regional Airport. The ground marshaller was seriously injured and the airplane received minor damage. The pilot said he was taxiing west on taxiway Delta when he saw the ground marshaller off his left wing. The ground marshaller waved him on and then ducked down. A witness said the marshaller made a 90-degree turn to tell the witness, who was on a motorcycle, where to lead the airplane. The airplanes left wing struck the ground marshaller in the back, knocking him forward. The left wing then struck the ground marshaller in the back of the head, knocking him to the ground.