Combinations of Beauveria bassiana (Balsamo) Vuillemin and five insecticide formulations were assessed for compatibility and efficacy for control of Colorado potato beetle (CPB), Leptinotarsa decemlineata (Say). In vitro testes with abamectin 0.15 emulsifiable concentrate, triflumuron 4 flowable, thuringiensin ABG-6162A(1.5% AI), and carbaryl 50 wettable powder demonstrated no significant inhibition of B. bassiana colony growth. In bioassays with neonate CPB, effects of B. bassiana alone were extremely variable. Combinations of B. bassiana with a given insecticide were consistently more toxic than B. bassiana alone. In most instances, the concentration-response line for the combination was parallel to the line estimated for the fungus alone, and the toxicity of the combination relative to the fungus varied within narrow limits. This result indicates that the two treatments (B. bassiana and insecticide) were additive. In field cage tests of B. bassiana + insecticide, mortalities were generally greater than those caused by the individual agents, but synergistic interaction was not apparent.