The fungal entomopathogen, Beauveria bassiana, was isolated from a naturally infected tarnished plant bug, Lygus lineolaris, in Arkansas. In laboratory tests, this strain, ARSEF 3769, was highly infective to tarnished plant bug nymphs and adults. The LC50 for nymphs and adults was 9x104 and 8.4x104 spores per ml when mortality was recorded 7 days after treatment. In field tests, cotton plants treated with aqueous suspensions of B. bassiana at a rate of 5.8x107 spores per ml resulted in 88.8% and 100% mortality (n=143) in exposed L. lineolaris adults at 5 and 7 days after treatment, respectively, compared with 7.4% and 11.5% in the controls (n=150). Field tests in 1995 with the commercial B. bassiana product, Mycotrol, and the insecticide imidacloprid, resulted in 97.9% mortality at 5 days after treatment in L. lineolaris adults when Mycotrol (280 g per ha) and Imidacloprid (50 g a.i. per ha) were applied together, compared with 9.5% mortality in the controls. The combination of Mycotrol and imidacloprid was significantly more effective than either material by itself. The fungus B. bassiana may useful for control of L. lineolaris in cotton.