The presence of an active cyclophorase system capable of oxidizing the principal acids of the Krebs acid cycle was demonstrated in mitochondria from hypocotyls and cotyledons of tomato seedlings. The optimum conditions for active mitochondrial preparations and maximum enzyme activity, especially for the succinoxidase system, were determined and are reported.Lycomarasmin and fusaric acid, two toxins produced by Fusarium oxysporum f. lycopersici, inhibit the succinoxidase and cytochrome oxidase activity of the mitochondria at a concentration of 10−2 M. The inhibition was overcome by the addition of catalytic amounts of cytochrome c. No effect was observed on succinic dehydrogenase. It was concluded that the toxins inhibit enzyme activity by affecting the integrity of the mitochondria allowing diffusion of cytochrome c from active sites. The role of the toxins in Fusarium wilt of tomatoes is discussed.