Abstract: Laboratory studies showed that three common fungicides, Bayleton, Cercobin-M and Derosal had adverse effects on Phytqseiulus persimilis, a predator of red spider mites. Bayleton showed a repellent effect on the predator. The percent settlement of P. persimilis was 21.7% on the fungicide treated plants of snap bean (Phaseolus vulgaris), comparing to 53.3% on the untreated one. When P. persimilis was placed onto a treated leaf with the other leaf untreated on the same plant, 80.8% of the predator escaped to the untreated leaf, only 19.2% remained on the treated. On the plants with both leaves untreated, 80% of the predators stayed. Cercobin-M and Derosal were found to be inhibitory to egg-laying of P. persimilis. After preying upon the spider raitss or even " the eggs laid by the mites fed on the treated plants, the predator suffered non-recoverablly reduced longevity and reproductivity. The inhibitory effect by Derosal remained active 7 days after the treatment. No harmful effect to P. persimilis was found by a bio-fungicide Agricultural Antibiotic 120.