The efficacy of Naturalis-L, a commercial formulation of Beauveria bassiana produced by Fermone Corp., for control of boll weevil, Anthonomus grandis grandis Boheman, was evaluated in laboratory and field experiments at two locations in Mississippi during 1992, and one location during 1993. Naturalis-L demonstrated some reduction of boll weevil damaged squares in field experiments, but the level of weevil control was significantly (P<0.05) less than that provided by Guthion (azinphosmethyl), a standard insecticide treatment. Beneficial arthropod populations were higher in the untreated check and the Naturalis-L plots as compared to those in the Guthion treated plots. Aphid populations and plant bug populations were reduced in the Naturalis-L treated plots. Additional laboratory experiments demonstrated that boll weevils were susceptible to the fungus and that significant mortality from the fungus can be obtained by 4 to 5 days post exposure. However, the level of mortality obtained was significantly less than that of Guthion treatment, and mortality of adult boll weevils exposed to Guthion occurred much faster with maximum mortality levels observable at 12 hours post treatment. Naturalis-L caused no significant mortality of boll weevils in a caged experiment with field treated plants. Current efficacy of Naturalis-L against boll weevil is not comparable to that obtained with standard chemical insecticides. Further research is needed to fully understand the potential use of this biological insecticide in cotton IPM.