Some properties of the citrate synthase from Chloroflexus aurantiacus have been examined in crude cell‐free extracts and partially purified preparations. The enzyme had an approximate native molecular size of 140 000, was not inhibited by NADH or 2‐oxoglutarate but was inhibited by ATP (about 50% at 5 mM). The Km for acetyl CoA at pH 8.2 in the presence of 0.5 mM oxaloacetate was determined to be 25 μM. These properties are characteristic of the ‘small’ size class of citrate synthases normally associated with gram‐positive eubacteria, despite the fact that Chloroflexus stains gram‐negatively.