A Phytoseiulus persimilis A.-H. race, resistant to the organophosphorous pesticides, has been developed by using the large-scale selection method and varying doses of pesticides to produce the necessary breeding effect. The resistance of this race is between 76.4 to 261.9 times higher than the one of the susceptible race and depends on the active composition and the formulation of the pesticides. The acquired resistance has been observed to affect in any way the ontogenetic development and the biologic indices of Ph. persimilis. With respect to some indices of economic importance the resistant race demonstrates 4 times higher effectiveness when used for biologic control against spider mites on the crops.