Dieldrin does not react with wool but, during treatment in aqueous emulsions, some of the insecticide penetrates the fibers. This small amount, sufficient to mothproof the wool, is not removed by the usual drycleaning solvents which do not cause swelling, but can be extracted completely by hot methanol or acetone. Dieldrin is gradually lost when treated wool is washed in soap solutions, but an initial application of 0.05% on the weight of wool gives a mothproofing effect which withstands machine washing for 3 hr. ASTM Standard Tests have been applied to wool containing 0.05% Dieldrin, and the insectproofing effect found to be fast (Class 3) to laundering, dry and wet cleaning, hot pressing, sea water, acid and alkaline perspiration, light, and rubbing. Dieldrin has been applied to large amounts of wool at various stages of processing: the results of these industrial trials are given. Treatment should preferably be in the final wet process, since some of the insecticide is removed by subsequent dyeing, bleaching, or milling. Application is preferred from the dyebath when dyeing is the final wet process.