Temperature-insensitive transformants that contained simian virus 40 sequences at only one or a few sites in the rat chromosome and that were induced by a temperature-sensitive A gene mutant of simian virus 40 were used to select flat revertants (revertants that had lost the transformed phenotype). The isolation was performed at the nonpermissive temperature so as not to select against temperature-sensitive transformants. Nonetheless, all of the revertants examined had lost their ability to express the T-antigen at both temperatures, and all contained rearrangements of the integrated simian virus 40 sequences. These results are most compatible with the hypothesis that the T-antigen of simian virus 40 is required for the maintenance of the transformed state even in temperature-insensitive cell lines.