The pilot intended to spray several crops in the same general area and noted that the first crop had power line obstructions. The spraying of this crop was completed except for clean-up runs and the pilot returned to carry out these runs after treating the other crops. On the first run the pilot passed abeam a power pole and thought he had flown under both power lines. However, as he began to pull up at the end of the crop the pilot saw a power line ahead and was unable to prevent the aircraft colliding with it. The aircraft remained controllable and the pilot made a normal landing at the destination strip.