While maneuvering during an aerial application flight, the engine lost power, and the pilot conducted an autorotation to a field. During the autorotation, the helicopter landed hard, rolled over, and sustained substantial damage. Postaccident examination of the helicopter revealed a foamy cream-colored liquid in the airframe fuel filter and fuel line. The pilot reported that before the accident flight the helicopter was fueled from the operator’s support truck fuel tank. Initial examination of the tank showed no visible signs of fuel contamination. A subsequent visual examination, several days later, showed the same cream-colored contamination that was identified in the helicopter fuel system. The pilot reported that the truck’s fuel tank was filled the day before from his on-site fuel storage tank. The reason for the fuel contamination in the operator’s on-site fuel storage or the helicopter fuel system could not be determined. The loss of engine power was attributed to fuel contamination.