Citrate synthases from diverse organisms are inhibited by ATP and NADH. Evidence is presented, from multiple-inhibition studies on various citrate synthases, that ATP acts in all cases as an isosteric inhibitor at the acetyl-CoA site. On the other hand, NADH also acts isosterically with eukaryotic and Gram-positive bacterial citrate synthases, but behaves as an allosteric inhibitor specifically in the case of the Gram-negative bacterial enzyme. After desensitization to this allosteric inhibition, only the isosteric nucleotide inhibition, as found in other citrate syntheases, is observed.