Meselson-Stahl density shift experiments have been used extensively to study selection and timing of plasmid replication. Experiments with plasmid R1 were previously performed and the conclusion was that this plasmid replicates one copy at a time and that there is an eclipse period after each replication during which no further replications can take place in the cell (Nordström et al., Plasmid 1, 187-203 (1978)). However, this interpretation is in conflict with other data, mainly with those obtained in copy number shift experiments (Gustafsson and Nordström, J. Bacteriol. 141, 106-110 (1980)). However, the density shift experiments have now been reinterpreted such that there no longer is any conflict with the copy number shift experiments. There does not seem to be any such eclipse period, but newly replicated plasmid molecules are not available for a second replication for about 20% of a generation time.