Bordetella dermonecrotic toxin (DNT) is known to activate the small GTPase Rho through deamidation or polyamination. In this study, we examined whether Rac and Cdc42, the two other members of the Rho family, serve as intracellular targets for the toxin. Immunoprecipitation and immunoblot assays revealed that DNT deamidated or polyaminated intracellular Rac and Cdc42. After the modifications, both Rac and Cdc42 lost their GTP-hydrolyzing, but not GTP-binding, activities. The interactions of the modified Rac and Cdc42 with their respective effectors were strictly dependent on GTP. MC3T3-E1 cells treated with DNT at high concentrations demonstrated extensive formations of lamellipodia and filopodia, which indicate the intracellular activation of Rac and Cdc42, respectively.