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Populations of Neoseiulus fallacis (Garman) and Phytoseiulus persimilis Athias-Henriot, predators of Tetranychus urticae Koch, were reared under homogeneous environmental conditions by inbreeding without introducing new germ plasm.Mites from 1-year-old inbred cultures had high mortality in all stadia and generally lower viability. Oviposition was reduced and a high percentage of eggs failed to hatch. Further inbreeding resulted in the production of only sterile female mites. These data cast serious doubt on the possibility of genetically improving populations of predaceous mites with only limited germ plasm resources.