Abstract: Some evidences showed difference in virulence of populations of Beauveriabassiana due to the different types of germination of conidia: unidirectional, bidirectional andmultidirectional. In CREA-ABP laboratory, some studies were finalized to evaluate theeffects of two strains of the fungus Beauveria bassiana, named Bb1 and Bb2, provided by acommercial producer, on the phytophagous mite Tetranychus urticae and on the phytoseiidNeoseiulus californicus. Treatments were performed following the indication of the producerand the IOBC guidelines. The fungus strains, after standard dilution and inoculation in Petridish, were checked, at different times from the inoculation, for the conidia concentration andtheir mode of germination (uni-, multi-directional). The following responses were evaluated:for five days, the direct toxicity on females and eggs of T. urticae and of N. californicus; theeffect on fertility of tetranychid and phytoseiid females, the escaping rate of the mites. Bothstrains of the fungus caused more than 50% of mortality on the tetranychid females. TheBb2 strain, the one with higher portion of unipolar germination, determined higher mortality(about 25%) in the phytoseiids females. Egg mortality was always limited: about 10%, thehighest value, was caused by Bb2 for tetranychid eggs. Furthermore, the same strain ofB. bassiana lowered the fertility, after two days, of the tetranychid females. It is discussedgermination polarity as potential tool, with several others, to indicate the pathogenic ability.