At a constant temperature of 24° C. the final larval instar ofDiataraxia oleracealasts about 10 days, during which its resistance to DDT and γ‐BHC as contact insecticides progressively increases up to the 5th or 6th day. It then suddenly decreases, this coinciding with cessation of feeding and the beginning of prepupal formation.Between the 2nd and the 6th days the gross body weight of the last‐instar larva increases from about 0.27 to 0.65g. Under the conditions of the experiments, the LD50 of parathion, as a stomach poison, was linearly related to body weight; on the same basis TEPP was slightly less, and lead arsenate, slightly more, toxic to the larger than to the smaller larvae. However, DDT as a stomach or contact insecticide, and γ‐BHC as a stomach poison were notably less toxic to the larger larvae. For example, the increase in LD50 for an increase in larval body weight of × 2 was about × 11 for DDT as a stomach poison and about × 12 as a contact insecticide.The order of effectiveness of the above insecticides as stomach poisons for the last‐instar larva ofD. oleraceawas parathion>DDT>γ‐BHC>TEPP = lead arsenate. Zinc fluoarsenate and rotenone were relatively non‐toxic. Larvae ofD. oleraceawere repelled by food leaf treated with an extract of