Streptomyces coelicolor A3(2) gives rise to spontaneous chloramphenicol sensitive mutants at a frequency of about 0.3% per spore. These mutants are often genetically unstable and give rise to arginine auxotrophs (Arg-) at frequencies of 1-7% per spore. These Arg- mutants usually lack the enzyme argininosuccinate synthetase (one exception was found that lacked ornithine carbamoyltransferase) and were shown to have deleted the corresponding argG gene by hybridisation analysis using a cloned S. cattleya argG gene. The Arg- strains also showed a variety of different DNA amplification and deletion events in a region homologous to an amplified DNA sequence found in spontaneous Arg- mutants in S. lividans 66.