Formation of the body of vertebrate embryos proceeds sequentially by posterior addition of tissues from the tail bud. Cells of the tail bud and the posterior presomitic mesoderm, which control posterior elongation(1), exhibit a high level of aerobic glycolysis that is reminiscent of the metabolic status of cancer cells experiencing the Warburg effect(2,3). Glycolytic activity downstream of fibroblast growth factor controls WNT signalling in the tail bud(3). In the neuromesodermal precursors of the tail bud(4), WNT signalling promotes the mesodermal fate that is required for sustained axial elongation, at the expense of the neural fate(3,5). How glycolysis regulates WNT signalling in the tail bud is currently unknown. Here we used chicken embryos and human tail bud-like cells differentiated in vitro from induced pluripotent stem cells to show that these cells exhibit an inverted pH gradient, with the extracellular pH lower than the intracellular pH, as observed in cancer cells(6). Our data suggest that glycolysis increases extrusion of lactate coupled to protons via the monocarboxylate symporters. This contributes to elevating the intracellular pH in these cells, which creates a favourable chemical environment for non-enzymatic beta -catenin acetylation downstream of WNT signalling. As acetylated beta -catenin promotes mesodermal rather than neural fate(7), this ultimately leads to activation of mesodermal transcriptional WNT targets and specification of the paraxial mesoderm in tail bud precursors. Our work supports the notion that some tumour cells reactivate a developmental metabolic programme. The authors show that metabolic activity leads to an increase in the intracellular pH of neuromesodermal precursors, and that this increase in pH, by allowing post-translational modification of beta -catenin, is required for the activation of WNT signalling and mesodermal fate acquisition.