Alkaline phosphatases from vegetative and sporulating cells of B. subtilis have been shown previously to be identical in all criteria examined. Despite this, 15 mutants producing low levels of the phosphatase during phosphate starvation of vegetative cells have been shown to produce high levels of the sporulation-specific alkaline phosphatase. It has been shown by imrnunochemical means that seven of these mutants when starved of phosphate produce low levels of normal wild-type enzyme. The sporulation form of the enzyme from one mutant (P-l00) has been shown to be identical with the phosphatases from vegetative and sporulating cells of the wild type. It is proposed that all the mutants have regulatory defects in the control of the alkaline phosphatase from vegetative cells but nevertheless retain an intact structural gene for the enzyme and the control system for the phosphatase during sporulation.