Examination of Chlamydomonas and Euglena mitochondrial morphology in synchronized cells by high resolution fluorescence microscopy, serial ultra-thin sectionelectron microscopy, and computer graphic analysis has found dynamic morphological changes such as fusion, fission, and branching during the cell cycle. Giant Euglena and Chlamydomonas mitochondria are formed by fusion of smaller reticular mitochondria at specific stages of the cell cycle. Giant mitochondria formation is associated with a temporary reduction of respiratory function suggesting a role in the cell cycle for giant mitochondria other than their primary respiratory function[1,2,3]. We examined the localization of mitochondrial DNA in synchronized cells of Euglena gracilis Z [2]. Mitochondrial DNA was localized by fixation of cells, staining with an anti-DNA monoclonal antibody, immuno-electron microscopy, and computer graphic analysis. Cross sections of normal size reticulate mitochondria in cells sampled 4 hours after the onset of light showed only a few colloidal gold particles confined to specific regions of the matrix while in other mitochondria, DNA molecules were not observed. Serial ultra-thin sectioning and immuno-electron microscopy found colloidal gold particles homogeneously distributed throughout the matrix of giant mitochondria. This demonstrates for the first time the distribution of mitochondrial DNA throughout the matrix of giant mitochondria suggesting that individual DNA molecules are not spatially separated as found in reticulate mitochondria. If mitochondria differ in their genetic composition, the temporary formation of giant mitochondrion through mitochondrial fusion would provide an opportunity during the cell cycle for gene exchange between individual mitochondrial DNA molecules and for mitochondrial DNA replication prior to mitochondrial division.