Abstract: This paper reported:1.The 60Co-γ radiation induced pyrethroid-resistance of predatory mite,Amblyseius nicholsi,an important natural enemy of citrus red mite Panonychus citri.2.Some other biological effects of 60Co-γ radiation on A.nicholsi.The LC50s of S-5439 (a pyrethroid insecticide) to the two strains,FR2 and PFR which were radiated three times with 2000 rad (dosage rate=140 rad/min) of γ-ray and selected with S-5439 for 12 months,were 1.3347×10-3 and 1.4307×10-3 ppm respectively,and were about 23-fold compared with that of the sensitive strain.And the selecting periods of FR2 and PFR were shorter than half that of FR1 and PDFR which were selected only with S-5439 for 27 months.FR1,FR2 and PDFR were all performed some extent cross-resistance to other pyrethroid insecticides.The LC50s of the tested pyrethroids to FR1,FR2 and PFR were higher than their application concentrations in fields.It will be desirable for the practical utilization of these resistant strains in biological control of pest mites.The results of radiation genetics suggested that A.nicholsi might be a parahaploid species.The increase of 60Co-γ dosage resulted in the decrease of sexual ration (♀:♂).The RLD50s (Medium Radiation Lethal Dosage) of F1 eggs estimated from the obtained data were different in the two cases of radiating only male father or both male and female parents.