Dimerization of multicopy plasmids is widely assumed to be disadvantageous both for plasmid maintenance and for the host cell. It is known that dimerization causes plasmid instability; dimer-containing cells grow slower than their monomer-containing counterparts. However, as we demonstrate here, under conditions of selective stress, dimers provide an advantage for bacteria. Dimers facilitate segregation of mutants from numerous copies of the parental plasmid. Accelerated segregation greatly increases the rate of accumulation of plasmids carrying mutations that are adaptive for bacteria. In contrast, resolution of dimers by site-specific recombination decreases, 103-105-fold, the efficiency of selection of spontaneous reversions in the tet gene of pBR327.