Two laboratory lines of Musca domestica from different sources and the Ellenville line were tested by exposure 5 or 6 days after emergence to residues of insecticides applied to plywood squares. The Ellenville line was not apparently more resistant to certain chemicals than were flies of one of the laboratory lines but the avg. rate of mortality of flies of the Ellenville line ranged from 19.4 to 43.0% when exposed for 30 min. or for 1 or 2 hrs. to 8 brands of technical DDT applied at 144 mg./sq. ft. compared with 100 or near 100% mortality for one of the laboratory lines. Flies of the Ellenville line proved resistant still to technical DDT, to the pp[image]-DDT, and to methoxychlor in the 10th and 11th generations, could survive repeated knockdown; they lived for the usual lengths of time after exposure and deposited the usual number of fertile eggs which produced satisfactory cultures of vigorous larvae.