Mitochondria from avocado (Persea americana Mill, var. Fuerte and Hass) can be rapidly prepared at every stage of ripening using differential centrifugation and self-generated Percoll gradients. The procedure results in improved oxidative and phosphorylative properties, especially for mitochondria isolated from preclimacteric fruits.A gradual change in the buoyant density of avocado mitochondria takes place during ripening. Climacteric and postclimacteric avocado mitochondria have the same buoyant density as other plant mitochondria (potato, cauliflower), whereas mitochondria from preclimacteric fruit have a lower density. The transition in buoyant density occurs during the climacteric rise, and two populations of intact mitochondria (p = 1.060 and p = 1.075) can be separated at this stage. Evidence indicates that the difference in mitochondrial buoyant density between preclimacteric and postclimacteric mitochondria is likely due to interactions with soluble cytosolic components.