With this acute respiratory infection model, we demonstrated in vivo efficacy of phage BcepIL02 in decreasing bacterial den- sity and lung inflammation during infection. These positive treatment effects were not observed with either UV-inactivated phage or l phage, and BcepIL02 was ineffective against infection with B. cenocepacia K56-2, a strain resistant to BcepIL02. These findings indicate that active and specific phages are required for effective treatment.