On July 31, 2000, about 0813 mountain daylight time, an Air Tractor AT-301, N4330S, registered to the pilot and operated by Harris Spray, Incorporated, as a 14CFR137 aerial application flight, was substantially damaged after colliding with a power pole guy wire and terrain near Brockway, Montana. The commercial pilot sustained serious injuries. Visual meteorological conditions prevailed and no flight plan was filed for the local flight. The flight originated from Brockway, Montana approximately 15 minutes prior to the accident. During a routine aerial application flight, the aircraft collided with a power pole guy wire. The pilot stated that his spray path was parallel to a series of power transmission lines and that he did not see the guy wire prior to the collision. The guy wire severed the outboard section of the aircraft's right wing and aileron. The aircraft subsequently rolled to the right and collided with terrain. The aircraft came to rest, inverted, approximately 1,100 feet from the initial impact point. The pilot reported that there were no mechanical malfunctions or failures that contributed to the accident.