The human beta-catenin locus (CTNNB1) was mapped by in situ fluorescence analysis to band p21 on the short arm of chromosome 3, a region frequently affected by somatic alterations in a variety of tumors. PCR primers for the genomic amplification of beta-catenin sequences were selected on the basis of homology to exon 4 of the Drosophila armadillo gene. Analysis of a panel of somatic cell hybrids confirmed the localization of beta-catenin on human chromosome 3. Furthermore, exclusion mapping of three hybrids carrying defined fragments of the short arm of human chromosome 3 allowed us to determine the position of the CTNNB1 locus close to the marker D3S2 in 3p21.