Aims: To evaluate the role of a-phosphoglucomutase (alpha-Pgm) and phosphoglucose isomerase (Pgi) activities in growth rate, sugar-phosphates, UDP-sugars and lactate biosynthesis in Lactobacillus casei. Methods and Results: The pgm and pgi genes coding for alpha-Pgm and Pgi activities in L. casei BL23, respectively, were identified, cloned and shown to be functional by homologous overexpression. In MRS fermentation medium with glucose, overexpression of pgm gene in L. casei resulted in a growth rate reduced to 75% and glucose-6P levels reduced to 47%. By contrast, with lactose, the growth rate was raised to 119%. An increment of a-Pgm activity had no significant effect on UDP-sugar levels. Remarkably, Pgi overexpression in L. casei grown in lactose or galactose resulted in almost a double growth rate with respect to the control strain. The increased Pgi activity also resulted in glucose-6P levels reduced to 25 and 59% of control strain cultured in glucose and lactose, respectively, and the fructose-6P levels were increased to 128% on glucose. UDP-glucose and UDP-galactose levels were reduced to 66 and 55%, respectively, of control strain levels cultured in galactose. In addition, the lactate yield increased to 115% in the strain overproducing Pgi grown in galactose. Conclusions: The physiological amount of alpha-Pgm and Pgi activities is limited for L. casei growth on lactose, and lactose and galactose, respectively, and that limitation was overcome by pgm and pgi gene overexpression. The increment of alpha-Pgm and Pgi activities, respectively, resulted in modified levels of sugar-phosphates, sugar-nucleotides and lactate showing the modulation capacity of the carbon fluxes in L. casei at the level of the glycolytic intermediate glucose-6P. Significance and Impact of the Study: Knowledge of the role of key enzymes in metabolic fluxes at the branching point between anabolic and catabolic pathways would allow a rational design of engineering strategies in L. casei.