Naturalis-L, which contains conidia of an insect-specific fungus, Beauveria bassiana (Balsamo) Vuillemin, was evaluated in a field test for activity against the tarnished plant bug, Lygus lineolaris (Palisot de Beauvois). Cotton infested with tarnished plant bugs received weekly applications of dimethoate (0.2 lb [AI]/acre) or Naturalis-L (15 oz/acre applied with 8 oz of sticker) during July 1993. Nymphs and adults were reduced by an average 53.8 and 20.2%, respectively, in the Naturalis-L treatment, and 68.9 and 59.6%, respectively, in the dimethoate treatment. This level of adult control by Naturalis-L is ineffective. However, in low early-season infestations 53.8% control of nymphs could be useful in maintaining populations below levels that require additional insecticide treatment.