Malathion was applied at 12 and 16 oz [A.I.]/a to cotton near Mercedes, TX, in 1995 and sampled on 0, 2, 4 and 6 d posttreatment to determine residues of malathion, malaoxon, and iso-malathion. Residues were then compared to determine toxicity to boll weevil, Anthonomus grandis Boheman. Residues of malathion comprised 99% of the leaf washes and 100% of the leaf homogenates. Malaoxon was found in one leaf wash sample of a leaf collected on 0 d from cotton sprayed with the low use rate. Iso-malathion was not found in any sample. Residues of 2.43 to 13.63 ppm malathion killed 100% of the weevils on d 0 and 2. Residues of 1.55 to 1.99 ppm killed 89% to 90% of the boll weevils on d 4 and 0.5 to 1.26 ppm killed 72% to 81% of the boll weevils on d 6.