A non-pigmented strain of (Db10) was isolated from moribund flies. From this strain were isolated spontaneous mutants resistant to streptomycin (Db11) and nalidixic acid (Db12). Mutant Db11 was used for the isolation of two phages, ⊘J and ⊘K, which grew on Db10, Db11 and Db12, but not on three reference strains of Mutant Db11 was demonstrated to fulfil Koch's postulates. Strain Db10 and its antibiotic-resistant derivatives were lethal to whether given in the food or by injection. Evidence for toxin(s) was found only in sterile supernatants from 7 d cultures. Such extracts contained proteolytic activity and inactivated the antibacterial activity in immune haemolymph from Phages ⊘J and ⊘K were used to isolate phage-resistant mutants of Db11. Three such mutants and their parental strain were investigated for their susceptibility to immune haemolymph from The parental strain was resistant to incubation with 90% haemolymph for 2h at 37βC; all phage-resistant mutants were susceptible to the immune haemolymph with ‘killing times’ (i.e. the time required to kill 90% of the viable cells) ranging from 15 to 55 min. When the same strains were compared for their virulence to , the phage-resistant mutants had significantly reduced virulence. It is concluded that resistance to insect immunity plays an important role in the overall pathogenicity of


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