Compared with growth on citrate, growth ofPseudomonasC12B with acetate as a carbon source resulted in approximately a four- to six-fold reduction in arylsulfatase activity irrespective of the sulfur source utilized to derepress arylsulfatase formation. Similarly, whenO-acetyl-L-serine, oxaloacetate, malate, or α-ketoglutarate served as carbon sources in place of acetate, a reduction in arylsulfatase levels also occurred to varying degrees Trivial explanations for these results have been ruled out but efforts to provide an explanation for the inhibitory effect of acetate were unsuccessful. Some evidence suggests that growth on acetate may generate the formation of a substance which inactivates or irreversibly inhibits the enzyme.