Abstract Glyoxylate and Krebs cycle marker enzymes cobanded on sucrose gradients after isopycnic centrifugation of mitochondrial pellets. Mitochondrial heterogeneity was investigated by treating particles with a Nitro Blue Tetrazolium reaction mixture designed to specifically increase the buoyant density of mitochondria containing the Krebs cycle enzyme isocitrate dehydrogenase. The banding of Krebs cycle enzymes at a heavier buoyant density was accompained by the glyoxylate cycle marker isocitrate lyase. Both isocitrate-metabolizing enzymes appear to be compartmented within the same mitochondria. This strongly suggests that regulation of one or both of these enzymes is necessary for directing carbon flow through the two different cycles.