The DNA content of individual mitochondria in rice root cells was analyzed by fluorescence microscopy. Differences in DNA content were detected between individual mitochondria. Some mitochondria contained no detectable nucleoid (DNA-protein complexes). The percent of mitochondria with DAPI(4',6-Diamidino-2-phenylindole) -stained nucleoids varied over the length of the root (root base, 33%; middle portion of root, 41%; root tip, 91%). The mean amounts of DNA per mitochondrial nucleoid were equivalent to 46.4 kbp in the root base, 52.0 kbp in the middle portion of root and 124.2 kbp in the root tip. The amount of DNA in individual mitochondria and the ratio of mitochondria with visible nucleoids were higher in the root tip than in other parts of the root. The estimated amount of DNA in almost all of the observed mitochondria was smaller than the amount of DNA equivalent to the rice mitochondrial genome size (490 kbp), even in root tip.