Medical implants are sometimes colonized by biofilm-forming bacteria, which are very difficult to treat effectively. The combination of gentamicin and ultrasonic exposure for 24 hours was previously shown to reduce the viability of Escherichia coli biofilms in vivo. This article shows that such treatment for 48 hours reduced viable E coli bacteria to nearly undetectable levels. However, when Pseudomonas aeruginosa biofilms were implanted and treated for 24 and 48 hours, no significant ultrasonic-enhanced reduction of viable bacteria was observed. The difference in response of these 2 organisms is attributed to greater impermeability and stability of the outer membrane of P aeruginosa.