Chlorpyrifos applied in-furrow at rates of 1 or 2 lb which AI/acre for protection of onion plants persisted in muck soil at levels above 1 ppm for 16 weeks. Granular formulations yielded slightly higher initial insecticide deposits disappeared at a slower rate than did emulsifiable concentrates. Chlorpyrifos residues in onion plants never exceeded 1 ppm and most of this was confined to the outer wrapper leaves.