Recessive mutants defective in the formation of non-specific alkaline phosphatase (EC 3.1.3.1) could be selected by staining colonies on a plate with p-nitrophenylphosphate after treatment with chloroform vapour. Since no complementation was observed among the nine mutants so far tested, all the mutations might occur in the same locus, phoH. The non-specific alkaline phosphatase was repressible, although a significant basal level of the enzyme activity was observed in the repressed condition.