We report the functional characterization of the galF gene of strain VW187 (Escherichia coli O7:K1), which encodes a polypeptide displaying structural features common to bacterial UDP‐glucose pyrophosphorylases, including the E. coli GalU protein. These enzymes catalyse a reversible reaction converting UTP and glucose‐1‐phosphate into UDP‐glucose and PPi. We show that, although the GalF protein is expressed in vivo, GalF‐expressing plasmids cannot complement the phenotype of a galU mutant and extracts from this mutant which only produces GalF are enzymatically inactive. In contrast, the presence of GalU and GalF proteins in the same cell‐free extract caused a significant reduction in the rate of pyrophosphorolysis (conversion of UDP‐glucose into glucose‐1‐phosphate) but no significant effect on the kinetics of synthesis of UDP‐glucose. The presence of GalF also increased the thermal stability of the enzyme in vitro. The effect of GalF in the biochemical properties of the UDP‐glucose pyrophosphorylase required the co‐synthesis of GalF and GalU, suggesting that they could interact as components of the oligomeric enzyme. The physical interaction of GalU and GalF was demonstrated in vivo by the co‐expression of both proteins as fusion products using a yeast two‐hybrid system. Furthermore, using a pair of galF +/galU + and galF/galU + isogenic strains, we demonstrated that the presence of GalF is associated with an increased concentration of intracellular UDP‐glucose as well as with an enhancement of the thermal stability of the UDP‐glucose pyrophosphorylase in vivo. We propose that GalF is a non‐catalytic subunit of the UDP‐glucose pyrophosphorylase modulating the enzyme activity to increase the formation of UDP‐glucose, and this function is important for bacterial adaptation to conditions of stress.