Deliberate enrichment of garden soil led to the isolation of anAchromobacter1 capable of lysing various Azotobacteriaceae on agar plates. Lysis was less pronounced in liquid media; attempts to demonstrate diffusible lytic factors were not successful. Thirty-four other microbes, freshly isolated from soils or obtained from collections, had no activity; two strains out of seven ofPseudomonas aeruginosa showed some degree of lytic action.