1. DNA from female and male Sphaerocarpos donnellii (liverwort) plants exhibits at least two species with buoyant densities of 1.703 (main band) and 1.691 (satellite) g cm(-3) in CsCl equilibrium gradients. At least part, if not all, of the satellite DNA is localized in plastids. It consists of up to 90% of uniformly sized circular molecules of an average circumference of 38.5 μm. Compared to other Chlorophyta, the liverwort's cpDNA is unusually low both in diensity and contour length. - 2. On the hand, cpDNA from the ferns Asplenium nidus and Pteris vittata resembles those of higher plants in buoyant density (1.697 g cm(-3)) and circumference (about 44.8 μm). - 3. Analysis of DNA from the archegoniate chloroplasts with restriction endonucleases indicates chat the cyclic molecules are monomers. - 4. The results show that the circular molecules found in cpDNA of higher plants do not represent the functionally required minimum size of DNA in plastids.