Bioassays were conducted to examine the pathogenicity of the fungus Beauveria bassiana (Balsamo) Vuillemin (Ascomycota: Hypocreales), strain NI8, against Lygus lineolaris (Palisot de Beauvois) and its impact on beneficial arthropods including Apis mellifera L., Crysoperla rufrilabris Burmeister, Orius insidiosus Say, Hippodamia convergens Guérin-Méneville, Harmonia axyridis (Pallas), Coleomegilla maculata De Geer, and field-collected Aranea spiders Salticidae and Thomisidae. Insects were treated with four concentrations of NI8 (3.9 × 10⁴, 2.3 × 10⁵, 4.2 × 10⁶, and 1.5 × 10⁷ spores/ml) directly via topical spray. Median lethal concentration (LC₅₀), sporulation response, and resistance ratio were estimated for all species except for the two groups of spiders. No significant differences in mortality (10 d after application) were observed among L. lineolaris, A. mellifera, and C. rufilabris, and all three species were highly affected when exposed to the highest concentrations of B. bassiana with 99.0, 98.2, and 90.0% mortality, respectively. Between 35 and 45% of the tested populations of O. insidiosus, H. convergens, field-collected crab spiders, and C. maculata were killed at 1.5 × 10⁷ spores/ml; whereas only 22 and 27%, respectively, of the field-collected jumping spiders and H. axyridis were killed at 10 d with the same concentration. No significant differences were found between the LC₅₀s measured for L. lineolaris and C. rufilabris 10 d after application. Results suggested that C. rufilabris would be highly affected by the NI8 strain of B. bassiana when applied for control of L. lineolaris. In contrast, B. bassiana appeared to have lower effects on the other beneficial arthropods assayed at the concentrations targeted for L. lineolaris control.