AbstractBoth meiotic and mitotic replication of chromosomal and cytoplasmic DNAs in a unicellular green alga,Chlamydomonas reinhardi, have been studied by isotopic transfer experiments coupled with density‐gradient centrifugation analysis. It has been shown that during the vegetative cycle (1) the replication mode of chloroplast DNA, as well as γ DNA, is semiconservative; (2) in synchronized culture these two cytoplasmic DNA satellites replicate coordinately with a high degree of synchrony; (3) the replication of chromosomal DNA is independent of and separable from that of the two cytoplasmic DNAs. It has been shown that during the sexual cycle (1) the chloroplast DNA replicates semiconservatively during zygote maturation, at which time there is neither chromosomal DNA replication nor nuclear division; (2) another DNA component (M‐band DNA) replicates extensively and appears in large quantity; (3) meiosis occurs during zygote germination and is accompanied by one round of semiconservative chromosomal DNA replication; (4) the M‐band DNA disappears in the early germination period, and the degraded substance is not incorporated into the newly replicated chromosomal DNA. A possible origin of the M‐band DNA and a few properties of the cytoplasmic DNA satellites have been elucidated. Genetic recombination and regulation of incoordinate replication in an eucaryotic system have been discussed in terms of the data