Abstract In this paper, we report the cloning of a Xenopus bHLH/PAS factor homologous to the mammalian aryl hydrocarbon receptor nuclear translocator (Arnt) or Drosophila Tango gene. Sequence data analysis indicates that protein domains organization in xArnt is strongly conserved and that xArnt is highly related to the mammalian Arnt1 isoform. As revealed by reverse transcriptase polymerase chain reaction and whole-mount in situ hybridization, xArnt gene is expressed during early and late development. At early stages, xArnt transcripts are restricted to the ectoderm and extends to the marginal zone at gastrula stage. In tail bud embryo, xArnt is strongly expressed in branchial arches, optical and otical vesicles, and pronephros and pronephritic duct.