The time sequence of DNA replication in successive mitotic cycles was investigated using synchronous cultures of the slime mold Physarum polycephalum. DNA molecules replicated in a small subfraction (about one fifth) of the S-period of one interphase were again replicated in the corresponding subfraction of the S-period in the following interphase. This suggests that the time of replication of a specific DNA molecule is genetically fixed in eukaryotic cells. © 1969.