alpha-Phosphoglucomutase (alpha-PGM) plays an important role in carbohydrate metabolism by catalyzing the reversible conversion of alpha-glucose 1-phosphate to glucose 6-phosphate. Isolation of alpha-PGM activity from cell extracts of Lactococcus lactis strain MG1363 led to the conclusion that this activity is encoded by yfgH, herein renamed pgmH. Its gene product has no sequence homology to proteins in the alpha-d-phosphohexomutase superfamily and is instead related to the eukaryotic phosphomannomutases within the haloacid dehalogenase superfamily. In contrast to known bacterial alpha-PGMs, this 28-kDa enzyme is highly specific for alpha-glucose 1-phosphate and glucose 6-phosphate and showed no activity for mannose phosphate. To elucidate the function of pgmH, the metabolism of glucose and galactose was characterized in mutants overproducing or with a deficiency of alpha-PGM activity. Overproduction of alpha-PGM led to increased glycolytic flux and growth rate on galactose. Despite several attempts, we failed to obtain a deletion mutant of pgmH. The essentiality of this gene was proven by using a conditional knock-out strain in which a native copy of the gene was provided in trans under the control of the nisin promoter. Growth of this strain was severely impaired when alpha-PGM activity was below the control level. We show that the novel L. lactis alpha-PGM is the only enzyme that mediates the interconversion of alpha-glucose 1-phosphate to glucose 6-phosphate and is essential for growth.