The interpretation of surveys of DDT-resistance in body-lice is handicapped by the fact that the genetics of this form of resistance have not been studied. This paper reports on a preliminary investigation on normal and resistant strains of lice, F(1) and F(2) hybrids of a mass cross and F(1) x R hybrids exposed to DDT dust and DDT-impregnated paper of various concentrations. The results were consistent with inheritance by a single gene pair (or possibly a small number of genes). The F(1) hybrids were intermediate in tolerance but were all killed by 5% DDT dust. This confirms a suggestion that 5% DDT dust would be suitable for testing for incipient resistance.