A new phage-host system is described in which phage α 3a grows on stationary phase Achromobacter mutant strains. Characteristic clear plaques are formed, at an e.o.p. 10-1 to 10-2, on already confluent bacterial lawns of the mutant strains. Phage growth is sensitive to aeration and growth only occurs under micro-aerophilic conditions. Lysates prepared on the mutant strains cannot transduce in contrast to transducing lysates prepared from wild type Achromobacter strains.