Aerial application tests of malathion ULV mixed in once-refined cottonseed (CSO) and horticultural mineral oils (HMO) were conducted in 1998 and 1999 to determine toxicity to boll weevils with leaf bioassays. The HMO's used in these tests were: Orchex^® 796, and WS2908, an Orchex^® 796 formulation with drift retardant. Mortalities showed that a 6 oz/A rate of malathion ULV mixed with 26 oz/A of CSO was as effective against boll weevils as a 12 oz/A rate of undiluted malathion ULV. In July and August 1999, efficacy of 6:26, 6:10, 7:18, 8:8, and 10:0 oz/A ratios of malathion ULV:CSO against boll weevils was determined. Results showed that 6 and 8 oz/A rates of malathion ULV can be as effective as an undiluted 10 oz/A rate for the first 2 - 3 days after treatment if the volume of the application is sufficiently high. Malathion residues accumulated on leaf surfaces during rain free periods in August after repeated applications of both undiluted and malathion ULV diluted in CSO.