Gene expression is regulated by different transcriptional regulators. The transcriptional regulator isocitrate lyase regulator (IclR) of Escherichia coli represses the expression of the aceBAK operon that codes for the glyoxylate pathway enzymes. In this study, physiological and metabolic responses of the deletion of the ic1R gene in E. coli BW25113 were investigated based on the quantification and analysis of intracellular metabolic fluxes. The knockout of the iclR gene resulted in a decrease in the growth rate, glucose uptake rate and the acetate secretion rate, but a slight increase in biomass yield. The latter could be attributed to the lowered metabolic fluxes through several CO2 generating pathways, including the redirection of 33% of isocitrate directly to succinate and malate without CO2 production as well as the reduced flux through the pentose phosphate pathway. Furthermore, although the glyoxylate shunt was activated in the iclR mutant, the flux through phosphoenolpyruvate (PEP) carboxykinase kept almost unchanged, implying an inactive PEP-glyoxylate cycle and no extra loss of carbon atoms in the mutant strain. Both the reduced glucose uptake rate and the active glyoxylate shunt were responsible for the minor decrease in acetate secretion in the ic1R knockout strain compared to that in the wild-type E. coli strain.