SUMMARYStreptomyces coelicolor ~3 ( 2 ) and S. lividans 66, which lack chloramphenicol acetyltransferase, gave rise to chloramphenicol-sensitive (Cmls) variants spontaneously at frequencies of 0.5 to 2 %. The fertility type of S. coelicolor in respect of the SCPI plasmid (SCPI+, SCPI-or NF) had no effect on chloramphenicol sensitivity or on the frequency at which Cml' variants arose.Cmls isolates spontaneously reverted to Cml" at frequencies one to three orders of magnitude lower than the frequency with which CmlS strains arose from Cml". Cml" revertants obtained spontaneously from CmlS clones again produced Cmls isolates at the normal frequency of several per cent. Therefore, CmlS and CmlR are reversible phenotypes.