Selective toxicity of carbophenothion and trichlorfon to the honey bee,A pis melliferaL., and the alfalfa leafcutting bee,Megachile rotllndataF., was established in dosage-mortality studies using the topical drop method. LD30values of trichlorfon for the honey bee, the male leafcutting bee, and the female leafcutting bee were 28.5, 250.0, and 515.0 μg/g, respectively. LD30values for carbophenothion were 12.6, 4.3, and 6.2 μg/g. Trichlorfon was 18–34 times more toxic to the honey bee than to the female leafcutter at LD30and LD95. Carbophenothion was 2–3 times more toxic to the leafcutting bee at both dosage levels. The differential toxicity of trichlorfon may be related to pH of the body fluid, which was 6.0 for the honey bee and 6.8 for the leafcutting bee. Trichlorfon is known to be highly unstable at neutral or higher pH levels. Trichlorfon at the topical LD50levels was a stronger acetylcholinesterase inhibitor than carbophcnothion to both test insects.