AbstractFermenter studies under batch and fed‐batch conditions were carried out to test the possibility of plasmid pBR322 production in large amounts by usingE. coli relAstrains. High amplification rates of pBR322 plasmid DNA were observed inE. coliCP79 (relA) andE. coliCP143 (relA) in both batch and fed‐batch cultivation after exhaustion of the amino acid arginine. The concentrations of plasmid DNA per unit of biomass were nearly the same in batch and in fedbatch fermentations ofE. coliCP79 andE. coliCP143. Therefore, the significantly higher biomass concentration of the two strains after fed‐batch fermentation gave a dramatic increase in the yield of plasmid DNA per litre of medium in comparison to the batch process.The results support the suggestion thatE. coli relAstrains are suitable hosts for production of large amounts of ColE1‐derived plasmids for recombinant DNA r