Wnt signalling regulates β-catenin-dependent developmental processes through the Dishevelled protein (Dsh). Dsh regulates two distinct pathways, one mediated by β-catenin and the other by Jun kinase (JNK). We have purified a Dsh-associated kinase from Drosophila that encodes a homologue of Caenorhabditis elegans PAR-1, a known determinant of polarity during asymmetric cell divisions. Treating cells with Wnt increases endogenous PAR-1 activity coincident with Dsh phosphorylation. PAR-1 potentiates Wnt activation of the β-catenin pathway but blocks the JNK pathway. Suppressing endogenous PAR-1 function inhibits Wnt signalling through β-catenin in mammalian cells, and Xenopus and Drosophila embryos. PAR-1 seems to be a positive regulator of the β-catenin pathway and an inhibitor of the JNK pathway. These findings show that PAR-1, a regulator of polarity, is also a modulator of Wnt–β-catenin signalling, indicating a link between two important developmental pathways.