Aims: To isolate and characterize novel bacteriophages for the phytopathogen, Erwinia carotovora ssp. atroseptica (Eca), and to isolate phage-resistant mutants attenuated in virulence. Methods and Results: A novel flagellatropic phage was isolated on the potato-rotting bacterial species, Eca, and characterized using electron microscopy and restriction analysis. The phage, named Theta AT1, has an icosahedral head and a long, contractile tail; it belongs to the Myoviridae family. Partial sequencing revealed the presence of genes with homology to those of coliphages T4, T7 and Mu. Phage-resistant transposon mutants of Eca were isolated and studied in vitro for a number of virulence-related phenotypes; only motility was found to be affected. In vivo tuber rotting assays showed that these mutants were attenuated in virulence, presumably because the infection is unable to spread from the initial site of inoculation. Conclusions: The Eca flagellum can act as a receptor for Theta AT1 infection, and resistant mutants are enriched for motility and virulence defects. Significance and Impact of the Study: Theta AT1 is the first reported flagellatropic phage found to infect Eca and has enabled further study of the virulence of this economically important phytopathogen.