Bacteriophages are considered to be themost abundant biological entities on the planet. The Siphoviridae are themost com-monly encountered tailed phages and contain double-stranded DNA with an average genome size of~50 kb. This paper de-scribes the isolation fromfour different activated sludge plants of the phage RRH1, which is polyvalent, lysing ve Rhodococcus species. It has a capsid diameter of only~43 nm.Whole-genome sequencing of RRH1 revealed a novel circularly permuted DNA sequence (14,270 bp) carrying 20 putative open reading frames. The genome has amodular arrangement, as reported for those of most Siphoviridae phages, but appears to encode only structural proteins and carry a single lysis gene. All genes are transcribed in the same direction. RRH1 has the smallest genome yet of any described functional Siphoviridae phage.We demonstrate that lytic phage can be recovered fromtransforming naked DNA into its host bacterium, thusmaking it a potentially usefulmodel for studying gene function in phages.