Abstract: In the laboratory tests conducted in Beijing in 1991, leaves of common bean (Phaseolus vulgaris) were infested with spider mites (Tetranychus truncatus) and sprayed with six commonly used pesticides at sublethal concentrations (LC_0-6.7)to examine their impact on the oviposition, egg-hatching rate and settlement of the predacious mite, Phytoseiulus persimilis. The pesticides tested were hexythiazox (50ppm), flufenoxuron (50ppm), amitraz (2ppm), pyridaben (200ppm), biphenthrin (0.2ppm) and cyhalothrin (0.05ppm). The results showed that hexythiazox and flufenoxuron had no harmful effect on the oviposition and egg-hatching of the predator. Amitraz had no obvious effect on the oviposition of the predator, but reduced egg-hatching rate by 2/3 compared with that in the control. Pyridaben, biphenthrin and cyhalothrin reduced oviposition by 28.3%, 17.8% and 14.4%, respectively. In a greenhouse test, the settlement of P. persimilis on the bean plants was not influenced by hexythiazox, fluofenoxuron or amitraz, but by pyridaben, biphenthrin and cyhalothrin. The settlement rate was 85% on the control plants, but only 25% on the plants treated with pyridaben and 0% on those treated with either biphenthrin or cyhalothrin. These results indicated that hexythiazox and flufenoxuron were safe to P. persimilis, while pyridaben had relatively harmful effect and amitraz, biphenthrin and cyhalothrin were harmful to the predator.