On June 5, 2000, about 0930 Pacific daylight time, a Grumman-Schweizer G-164B (Ag Cat), N6549K, registered to and operated by Southeastern Aviation Incorporated, as a 14CFR137 aerial applicator, was substantially damaged after colliding with power lines and terrain 15 miles west of Colfax, Washington. The commercial pilot sustained serious injuries. Visual meteorological conditions prevailed and no flight plan was filed for the local flight. There was no report of fire. The flight originated from Whitman County Memorial Airport, Colfax, Washington, approximately 20 minutes prior to the accident. The pilot reported that he was conducting a routine aerial application flight. He stated that he was aware of the power line that crossed his flight path (perpendicular) and that he intended to fly under them. However, the aircraft collided with the power line and the aircraft subsequently collided with terrain. According to a deputy from the Whitman County Sheriff's Department, the power lines measured approximately 40 feet above ground level (AGL). The pilot reported that he had not experienced any mechanical difficulties with the aircraft that contributed to the accident.