Germline LKB1/STK11 mutations are associated with the cancer-prone Peutz–Jeghers syndrome (PJS) in humans^,,, and nullizygosity provokes a poorly understood constellation of developmental perturbations in the mid-gestational mouse. To gain a better understanding of the processes regulated by LKB1, we have exploited the experimental merits of the developing Xenopus embryo. Here, specific inhibition of XEEK1, the Xenopus orthologue of LKB1, engendered developmental anomalies — shortened body axis and defective dorsoanterior patterning — associated previously with aberrant Wnt signalling. In line with this, LKB1/XEEK1 cooperates with the Wnt–β-catenin signalling in axis induction and modulates the expression of Wnt-responsive genes in both Xenopus embryos and mammalian cells. We establish that LKB1/XEEK1 acts upstream of β-catenin in the Wnt–β-catenin pathway in vivo. LKB1/XEEK1 regulates glycogen synthase kinase (GSK)3β phosphorylation and it is physically associated in vivo with GSK3β and protein kinase C (PKC)-ζ, a known GSK3 kinase. These studies show that LKB1/XEEK1 is required for Wnt–β-catenin signalling in frogs and mammals and provides novel insights into its role in vertebrate developmental patterning and carcinogenesis.