The α-proteobacterium Rhodobacter capsulatus is a model organism for the study of bacterial photosynthesis and the bacteriophage-like gene transfer agent. Characterization of phages that infect Rhodobacter is extremely rare, and scarce for the α-proteobacteria in general. Here, we describe the discovery of the only functional Mu-like transposing phage to have been identified in the α-proteobacteria, RcapMu, resident in the genome-sequenced R. capsulatus SB1003 strain. RcapMu packages ~42kb of total DNA, including <3kb of host DNA with no conserved motifs, indicative of replicative transposition with little insertion site preference. The phage genome contains 58 ORFs with comparable organization to known transposable phages. Shotgun proteomics of purified RcapMu particles detected all proteins with predicted structural functions as well as seven hypothetical proteins. Overall, comparison of RcapMu to enterobacteria phage Mu and other Mu-like phages revealed only regional homology to these phages, providing further evidence for the promiscuous, modular nature of bacteriophage evolution.