The direct and residual toxicity of thuringiensin on different stages of Tetranychus urticae Koch and Panonychus ulmi (Koch), and the influence on T. urticae reproduction and population development were evaluated in laboratory bioassays. Direct toxicity was higher than residual toxicity to T. urticae for all life stages. Immature stages of T. urticae and P. ulmi were more susceptible to thuringiensin than adults. P. ulmi was more susceptible to thuringiensin than T. urticae. Fecundity of T. urticae was significantly reduced when females were exposed to residues for 2 days. Complete suppression of T. urticae population development was achieved when the F1 generation was exposed to thuringiensin residues, however, the level of suppression was concentration dependent. The results suggest that thuringiensin acts in a relatively short time. Therefore, the high toxicity of thuringiensin on immature spider mite stages and the sublethal effects on females suggest that thuringiensin may successfully control field populations. The use of thuringiensin in spider mite control programmes is discussed. Practical suggestions on the development of bioassays for active ingredients of this type are also discussed