Dosage mortality data by a topical application method for eight organophosphorus compounds are given for two laboratory-selected .strains of Blattella p:ermanica ( L.), one resistant to malathion and the other resistant to diazinon. Comparison of this data with similar data for a normal susceptible strain of the same species reveals that development of resistance to malathion in this species is highly specific imparting no effective cross resistance to any of the other seven compounds evaluated. Selection with diazinon induces a low level of tolerance to all the organophosphates studied, but there are indications that in most cases this is due to vigor tolerance rather than to true resistance. There is reason to believe that if carboxyesterase activity is responsible for the resistance phenomena to malathion exhibited by this insect, its action is blocked or its detoxifying ability rendered ineffective by the presence of a vinyl group in an organophosphorus compound, such as GC 3707. / Master of Science