Toxicological studies were undertaken with radioactive-labeled parathion, Bayer 22408 (O,O-diethyl O-naphthalimido phosphorothioate), and dimethoate to determine the dosage of these toxicants absorbed by larvae of Anopheles guadrimaculatus Say, Aedes aegypti (L.), Aedes taeniorhynchus (Wied.) The interrelationship of the dosage absorbed, the concentration, the time of exposure, and the mortality were also determined. Results indicated that the efficiency of uptake differed with the insecticide used. The concentration required to kill larvae did not necessarily reflect the amount of insecticide that entered the insect. For example, dimethonte in 24-hour exposure tests gave an LC-50 of 4.0 p.p.m. and an LD-50 of 0.0040 μg/larva; whereas Bayer 22408 gave an LC-50 of 0.009 p.p.m. and an LD-.50 of 0,0020 μ,g/larva. The amount of insecticide found in larvae after 24 hours of exposure at the LC-50 concentration was only a very small percentage of the total to which they were exposed: with dimethoate, 0.02%; with parathion, 0.7%; and with Bayer 22408, 2.2%. Better methods if treatment could make some inferior toxicants into effective larvicides and should increase the efficiency of all toxicants. All three insecticides were excreted readily by larvae.