A virulent phage (phiCR1) capable of generalized transduction in Citrobacter rodentium was isolated from the environment and characterized. C. rodentium is a natural pathogen of mice, causing transmissible murine colonic hyperplasia. Sequencing of its genome has recently been completed and will soon be fully annotated and published. C. rodentium is an important model organism for infections caused by the human pathogens enteropathogenic and enterohaemorrhagic Escherichia coli (EPEC and EHEC). phiCR1 uses a lipopolysaccharide receptor, has a genome size of approximately 300 kb, and is able to transduce a variety of markers. phiCR1 is the first reported transducing phage for C. rodentium and will be a useful tool for functional genomic analysis of this important natural murine pathogen.