Deletion of both iclR and arcA in E. coli profoundly alters the central metabolic fluxes and decreases acetate excretion by 70%. In this study we investigate the metabolic consequences of both deletions in E. coli BL21 (DE3). No significant differences in biomass yields, acetate yields, CO(2) yields and metabolic fluxes could be observed between the wild type strain E. coli BL21 (DE3) and the double-knockout strain E. coli BL21 (DE3) Delta arcA Delta iclR. This proves that arcA and iclR are poorly active in the BL21 wild type strain. Noteworthy, both strains co-assimilate glucose and acetate at high glucose concentrations (10-15 g l(-1)), while this was never observed in K12 strains. This implies that catabolite repression is less intense in BL21 strains compared to in E. coli K12.