On October 28, 1999, about 0915 hours Pacific daylight time, a Hiller UH-12E, N5374V, operated by R and B Helicopter, Inc., collided with a power line in Castroville, California. Thereafter, the helicopter impacted the ground and was substantially damaged. The commercial pilot was seriously injured. Visual meteorological conditions prevailed during the aerial application flight, which was performed under the provisions of 14 CFR Part 137. The flight originated about 0905 from an adjacent field. The pilot reported to the Safety Board investigator that he had been spraying in the general area, and had just altered his course to move away from telephone repair personnel who were working on a pole located near the field he intended to spray. Also, while flying between 25 and 30 feet above the ground, he had diverted his attention toward the global positioning satellite receiver in which he was to mark his new spray position. While proceeding over the field he looked up and observed a power line directly in front of his helicopter. There was insufficient time to turn. The pilot stated that he collided with the power line and lost control of the helicopter.