A modification of a method devised by Sternberg et al. (1954 Jour. Agric. and Food Chem. 1,125-30) made it possible to assay the DDT-dehydrochlorinase (DDT'ase) in individual flies of R population of DDT-resistant (R) house flies, Musca domemica L. By this means it was possible to obtain a measure of the variability of this enzyme in a population. It was also possible to relate the concentration of DDT'ase in parents with that found in the offspring. Regardless of the enzyme activity of the individual parents, that of the progeny appeared to drift toward a mean value. Flies selected for resistance to DDT alone have considerably ess DDT'ase than after being selected for resistance to a com- bination of DDT and DMC (bis-(p-chlorophenyl) methyl carbinol). A strain of flies selected for resistance to combination (DMC-R strain) was used to cross, inbreed and backcross with susceptible flies to determine the inheritance pattern of DDT'ase and its relationship to resistance. By this means a quantitative relationship between presence of DDT'ase and resistance to DDT has been established. Furthermore, the pattern of inheritance of resistance and DDT'ase activity shows no serious discrepancy with the proposal of Milani (1956, Riv. Parassitol., 17: 223--46), that resistance to DDT may be governed by a single partially dominant gene which behaves according to simple Mendelian principles.