F lac S is a thermostable extrachromosomal element isolated in Salmonella typhimurium which is altered in its replication as compared to its precursor F ts114 lac . Sedimentation of both these plasmids in alkaline sucrose gradients has indicated a difference in their sizes. Contour length measurements of open circular plasmid DNA molecules photographed in the electron microscope have revealed the estimated molecular weight of F ts114 lac to be 81 × 10 6 daltons while that of F lac S is 109 × 10 6 daltons. F lac S may carry a segment of S. typhimurium chromosomal or cryptic plasmid DNA.