The pilot was spraying a cotton crop with insecticide. Although he had not sprayed the field for about 12 months he remembered a power line traversing it, but could not remember if he had flown over or under the wire previously. On this occasion he elected to fly under it, however, as the aircraft passed under the wire it snagged the rudder. The pilot reported that the aircraft remained controllable but the rudder was jammed to the right. He slowed the aircraft to 70 knots to reduce the effectiveness of the rudder but was unable to reduce the extreme right yaw. He decided to return to the town airstrip where better facilities were available and advised the police of his problem by radio. He was subsequently able to land the aircraft without further incident. Inspection later revealed that the rudder horn had been severely damaged, jamming the rudder. The power line was reported to be 3 metres above ground level where the strike occurred.