SPINACH chloroplasts, like other higher plant chloroplasts, contain DNA1 and divide2. It has been estimated3–5 that chloroplasts contain many copies of DNA, but in higher plants, which contain large numbers of chloroplasts per cell2,6 it is not known when the DNA of chloroplasts replicates and how it is distributed when they divide. In the unicellular alga, Ochromonas, which contains a single chloroplast, 3H-thymidine is readily incorporated into chloroplast DNA and is distributed equally among the progeny chloroplasts3. On the other hand, in Acetabularia, which contains large numbers of chloroplasts per cell, DNA could not be detected in 65–80% of the plastids, but could easily be detected in the remainder7.