In small cages, residues of DDT knocked down all houseflies liberated therein from May 3 to Sept. 22 in 45-75 min.; in a large cage they knocked down all flies liberated therein from May 21 to Sept. 22 in 60-120 mins., and in a small dairy barn they knocked down all flies liberated therein from June 19 to Sept. 23 in from 75 to 120 min. No flies recovered in the foregoing tests. DDT residues effected a practical reduction of the fly population in a large dairy barn from June 29 to Oct. 8 and not only completely destroyed the fly population in 9 other treated barns but prevented an accumulation of flies following treatment.