Mycobacterium smegmatis was grown on Lemco agar in the presence and absence of acetate and fumarate. Fumarate was oxidized rapidly by fumarate-grown organisms. Lemco-grown and acetate-grown organisms oxidized fumarate at an increasing rate after a lag period and this adaptation was inhibited by chloramphenicol. Disrupted bacterial preparations oxidized fumarate rapidly, irrespective of the growth medium. Acetate-grown organisms oxidized acetate at more than twice the rate of fumarate-grown and Lemco-grown organisms. There was slight adaptation to acetate oxidation by acetate-grown organisms but greater adaptation by Lemco-grown and fumarate-grown organisms. Acetate was oxidized by disrupted bacterial preparations. The evidence for the existence of acetate and fumarate permeases is discussed.