Bacteriophages (or phages) are the most abundant biological entities on earth, and are estimated to outnumber their bacterial prey by tenfold(1). The constant threat of phage predation has led to the evolution of a broad range of bacterial immunity mechanisms that in turn result in the evolution of diverse phage immune evasion strategies, leading to a dynamic co-evolutionary arms race(2,3). Although bacterial innate immune mechanisms against phage abound, the only documented bacterial adaptive immune system is the CRISPR/Cas (clustered regularly interspaced short palindromic repeats/CRISPR-associated proteins) system, which provides sequence-specific protection from invading nudeic acids, including phage(4-11). Here we show a remarkable turn of events, in which a phage-encoded CRISPR/Cas system is used to counteract a phage inhibitory chromosomal island of the bacterial host. A successful lytic infection by the phage is dependent on sequence identity between CRISPR spacers and the target chromosomal island. In the absence of such targeting, the phage-encoded CRISPR/Cas system can acquire new spacers to evolve rapidly and ensure effective targeting of the chromosomal island to restore phage replication.