We have examined the association of an F' plasmid and an R plasmid in Proteus mirabilis with a rapidly sedimenting material that is generated by sodium dodecyl sulfate lysis and low speed centrifugation. Virtually all of the chromosomal deoxyribonucleic acid (DNA) and the F' plasmid DNA are associated with the rapidly sedimenting material after gentle lysis and centrifugation. A portion of R plasmid NR1 DNA (usually 5 to 25%) is not bound to the rapidly sedimenting material and is recovered in the supernatant fraction. This difference in binding is not related to the size of the plasmid DNA, since F' plasmids and R plasmids of different molecular weights showed the same behavior. R plasmid DNA labeled by a brief pulse of [(3)H]thymine is recovered in the supernatant fraction to a lower extent than the total R plasmid DNA. It would appear that R plasmid replication takes place in association with the rapidly sedimenting material. With prolongation of the [(3)H]thymine pulse, the [(3)H]thymine-labeled R plasmid DNA is recovered in the supernatant fraction with the same probability as the total R plasmid DNA. This finding indicates that a change in R plasmid attachment to the rapidly sedimenting material occurs some time after its replication. The differences observed in the replication of F' plasmids and R plasmids in P. mirabilis may be related to their different modes of association with the rapidly sedimenting material.