Effect of Bacteria strains were investigated against the tritonymphs of the so-called European house dust mite Dermatophagoides pteronyssinus (Trouessart). The tritonymphs exposed to Bacillus spp. spores mixed in the feeding medium at different concentrations showed different rate of toxicity. Bacillus sphaericus spores were more toxic than B. thuringiensis. Treating the tritonymphs with Bacillus spp. caused prolongation of the times needed for the development of adult stage.