On July 16, 1998, approximately 0800 mountain daylight time, a Rockwell S-2R agricultural aircraft, N5036X, registered to Kinney Aviation Inc. (d/b/a Farm Dusters) of Missoula, Montana, and a Grumman-Schweizer G-164B agricultural aircraft, N6649K, registered to Farm Flite Inc. of Burley, Idaho, collided in flight approximately 12 miles northwest of Burley, Idaho. Both aircraft subsequently impacted terrain. Both aircraft were substantially damaged, and the commercial pilot-in-command of each aircraft was fatally injured. Available information indicated that at the time of the collision, both aircraft were operating under 14 CFR 91, with N6649K loaded and heading to a job site and N5036X empty and returning from a job site. Visual meteorological conditions (clear skies and 30 miles visibility) were reported at Burley at 0748, and neither aircraft had filed a flight plan. There were no eyewitnesses to the accident, although a loud crash was heard by an individual who was about 1/2 mile west of the crash site. He stated that he "heard a loud bang, then the motor reave [sic] up. Then a second bang." Evidence developed during on-scene investigation by FAA inspectors indicated that at the time of the collision, the S-2R monoplane was heading generally northeast and the G-164B biplane was heading generally southeast. U.S. Naval Observatory astronomical data indicated that at Burley at 0800 on July 16, the azimuth of the sun was 076.9 degrees true, at an altitude of 17.6 degrees. A map with the relative locations of the fields undergoing aerial application, airstrips being used, and the wreckage locations is attached. It was reported that the G-164B pilot was using the Hoard private airstrip near Kimama Butte, and was en route to spray the Patterson field on 850 West street and 200 North; and the S-2R was returning from the Hunt field at 1450 West and 400 South to the Kinney private airstrip near Kimama, Idaho. Autopsies on both pilots were performed under the authority of the Minidoka County Coroner, Rupert, Idaho. The autopsy on the S-2R pilot was performed at the necropsy lab of Snake River Pathology Laboratory, Burley, Idaho, on July 16, 1998. The cause of the S-2R pilot's death was found to be internal injuries secondary to impact, with the manner of death classified as accidental. At the time this factual report was submitted, the NTSB had not received an autopsy report on the G-164B pilot, and the Minidoka County Coroner reported to the NTSB that he had not yet received the autopsy report on the G-164B pilot from the pathologist. Toxicological testing on both pilots was performed by the FAA Civil Aeromedical Institute (CAMI), Oklahoma City, Oklahoma. Toxicological testing of the S-2R pilot was negative for ethanol and drugs. Carbon monoxide and cyanide testing were not performed on the S-2R pilot due to lack of suitable specimens. Toxicological testing of the G-164B pilot was negative for ethanol, drugs, carbon monoxide, and cyanide.