In the colorless alga Polytoma obtusum , the leucoplast DNA which constitutes approximately 16% of whole cell DNA, has a buoyant density in CsCl of 1·682 g/cm 3 and a G+C content of 17·4% by base composition analysis. The unusually high A+T content of the leucoplast DNA was also reflected in its high phosphate (0·3 m ) elution characteristics on an hydroxyapatite column. The presence of considerable intramolecular heterogeneity of this leucoplast DNA was first indicated by its asymmetrical optical melting profile, which consisted of a major (80%) highly co-operative fraction and a minor (20%) gradual hyperchromic transition, with an overall melting temperature in 0·15 m -NaCl, 0·015 m -Na citrate, (pH 7·0) of 75·6°C. When sheared leucoplasT DNA was melted on an hydroxyapatite column, a more heterogeneous profile was obtained, indicating the presence of short G+C-rich fragments interspersed among A+T-rich sequences. This was further demonstrated by analysis of the G+C content of DNA samples taken from the peak region of sheared leucoplast DNA banded in CsCl. The central 25 to 30% of the DNA peak had an average G+C content of 15 to 18%, the lightest 15% had less than 12% G+C, while about 20% had a G+C content of over 30%. The G+C-rich fragments appeared to be covalently linked to the A+T-rich sequences, since they were only evident when the size of the leucoplast DNA was reduced to an M r value of less than 3×10 6 . Renaturation kinetic studies revealed the presence of three, classes of sequences: (1) the unique sequences exhibited a reassociation rate similar to that of T 4 DNA, and constituted about 63% of the total; (2) zero-time DNA constituted approximately 27% of the total; and (3) very rapidly renaturable sequences constituted the remaining 10% of leucoplast DNA, with, a reassociation rate-constant k 2 of 1250, or about 500 times faster than T 4 DNA. The kinetic complexity of the single leucoplast genome was calculated to be, equivalent to an M r value of 2·8×10 8 to 3·2×10 8 , and an estimated 56 to 65 repetitions of this genome were present per cell. The evolutionary affinity between the leucoplast of Polytoma and the chloroplast of Chlamdydomonas was assessed by sequence homology studies of their DNAs. Under our experimental conditions, 32% of the unique sequences of the leucoplast DNA hybridized with chloroplast DNA, while no appreciable hybridization between the plastid DNAs and T 4 DNA or DNA of Nostoc , a blue-green alga, was detected.