The effects of temperature, host plant, active ingredient mobility, surfactant, residue age and larval age on the response of Tetranychus urticae Koch to thuringiensin were evaluated in laboratory bioassays. T. urticae larvae were significantly more susceptible to thuringiensin at 13°C than at 28°C. A significantly different response of T. urticae larvae to thuringiensin residues was found on peach and apple leaves. Older T. urticae larvae were significantly more susceptible than younger larvae when exposed to thuringiensin residues. No systemic or translaminar effects of thuringiensin were detected; the leaf surface (upper or lower) and the addition of Silwet L-77 surfactant did not affect the efficacy of thuringiensin. The use of thuringiensin in spider mite control programmes is discussed. Practical suggestions on the development of bioassays for active ingredients to assess the effects of environmental factors of this type are also discussed.