Abstract: The toxicity of thuringiensin (orβ-toxin) produced by Bacillus thuringiensis (Bt) isolate CT-43 to the predatory mite, Phytoseiulus persimilis, and its prey, the two-spotted spider mite, Tetranychus urticae was tested in the laboratory in Beijing during 1992. At the 3rd-day after treatment, the LC₅₀ of the larvae of T. urticae and P. persimilis was 114.58 μg/ml and 211 μg/ml, respectively. Oviposition of both prey spider mite and the predatory mite was reduced by the thuringiensin treatment. The oviposition began to decline at the 2nd-day after the treatment, and ceased completely 3-4 days after the treatment. At the 5th-day after the treatment, the LC₅₀ of the adults of the prey and the predator was determined to be 279.88μg/ml and 843.19μg/ml, respectively. It is concluded that thuringiensin has a non-selective acaricidal effect. Although its toxicity to the phytoseiid predatory mite was lower than that to the two-spotted spider mite, indicating an obvious inhibiting effect on the predator\'s oviposition. It is thus suggested that use of thuringiensin should be avoided when release of P. persimilis is a necessary component of pest control programmes.