A field trial of ultralow volume spraying of synergized pyrethrins was carried out in a malarious coastal locality of El Salvador during the main transmission season of 1974. The spray was applied with ground equipment, and the insecticide used was a 5 percent solution of pyrethrins synergized with 15 percent piperonyl butoxide. Entomological evaluation of the spraying using mosquitoes in cages placed in and near houses, showed that the spraying caused high mortalities in these mosquitoes. Light trap studies in the sprayed and control localities showed a depression of the natural population of the vector Anopheles albimanus in the sprayed area. Malaria surveillance indicated that the ULV spraying had a marked impact on transmission of the disease during the main transmission season.