The distribution of mitochondria from Aspergillus nidulans on a dextran-sucrose gradient resulted in the fractionation of three bands. The lightest band presented the highest respiratory and cytochrome c oxidase activities and was also the most active in the incorporation of tritiated glycerol. Pulse-chase experiments with 3H-glycerol suggested that mitochondria from the light band was transformed to denser organelles as the organism grew.