After exhaustion of citrate in the growth medium, the citrate lyase of Rhodopseudomonas,gelatinosa is inactivated by deacetylation. Upon addition of citrate to a suspension of cells containing inactive enzyme the lyase is reactivated. The discovery of Buckel, Buschmeier and Eggerer (1), that deacetylation causes loss of activity of purified citrate lyase from Klebsiella,aerogenes, therefore, seems to be the basis of a regulatory mechanism of biological significance.