Methidathion resistance in Phytoseiulus persimilis was polyfactorially inherited. Twenty sub strains isolated from single pairs of susceptible mites and reared without selection showed a lower or a greater resistance level than the parent strain, and 10 other sub strains selected during seven generations displayed a significant increase in resistance. A direct selection for susceptibility of either susceptible or resistant strains was not possible. The nature of the genetic mechanism responsible for mediation resistance in P. persimilis is discussed, and the probable involvement of amplification of resistance genes is suggested.