The emphasis in phage research for many years has centred around phages specific to Escherichia coli and other members of the Enterobacteriaceae. Phages attacking other bacterial genera have not been so intensively studied until recent years. A diverse bacterial flora is known to inhabit natural environments such as soil and intensive investigation of soil micro-organisms for associated phages has been fruitful. Azotobacter phages were isolated by Duff and Wyss (1961), Pseudomonas phages were isolated by Bradley (1966) and twenty-eight distinct bacteriophages for various strains of Rhizobium trifolii were isolated by Barnet (1972). The present work was undertaken to investigate the number, nature and interrelationships of phages infecting B. pumilus.