The kinetic complexity of chloroplast DNA isolated from the chromophytic alga Olisthodiscus luteus has been determined. Using optical reassociation techniques, it was shown that the plastid DNA of this alga reacted as a single component with a second order rate constant of 4.1 molar(-1) and second(-1) (Cot((1/2)) 0.24 molar second) under conditions equivalent to 180 millimolar Na(+) and 60 degrees C. Given the 92 x 10(5) dalton complexity calculated for this chloroplast genome, an Olisthodiscus cell contains 650 plastome copies. Although this complement remains constant throughout the growth cycle of the organism, the ploidy level of an individual chloroplast shows significant plasticity and is dependent upon the number of chloroplasts present per cell. Experiments with the DNA fluorochrome Hoechst dye 33258 (bisbenzimide) demonstrate that plastids isolated from all phases of cell growth each possess a ring-shaped nucleoid containing detectable DNA. Olisthodiscus chloroplast DNA showed no sequence mismatch when thermal denaturation profiles of reassociated chloroplast DNA were examined, thus all plastome copies are essentially identical. Finally, reassociation studies demonstrated that no foldback (short inverted repeat) sequences were present in the plastid genome although significant hairpin loop structures were observed in control nuclear DNA samples.