In Klebsiella aerogenes, arylsulfatase synthesis was repressed by inorganic sulfate, sulfite, sulfide, thiosulfate, and cysteine, but not by methionine under normal growth conditions. We isolated cysteine-requiring mutants (Cys minus), and mutants (AtsS minus, AtsR minus) in which the regulation of arylsulfatase synthesis was altered. In the cysteine auxotroph, enzyme synthesis was also repressed by inorganic sulfate or cysteine. Kinetic studies on mutants of the cysteine auxotroph showed that inorganic sulfate repressed arylsulfatase synthesis and that this was not due to cysteine formed by reduction of sulfate. Arylsulfatase synthesis in the AtsS minus mutant was not repressed by inorganic sulfate but was repressed by cysteine. This mutant strain had a normal level of inorganic sulfate transport. Another mutant strain, defective in the inorganic sulfate transport system, synthesized arylsulfatase in the presence of inorganic sulfate but not in the presence of cysteine. The AtsS minus mutant could synthesize the enzyme in the presence of inorganic sulfate but not cysteine. The AtsR minus mutant could synthesize the enzyme in the presence of either inorganic sulfate or cysteine. These results suggest that there are two independent functional corepressors of arylsulfatase synthesis in K. aerogenes.