The aryl hydrocarbon receptor (AHR) is a member of the basic helix-loop-helix/Per-Arnt-Sim (bHLH/PAS) family of transcription factors. Although this receptor has been known to mediate the toxic effects of environmental pollutants, its physiological functions remain elusive. Here. we describe the isolation and expression pattern of the Xenopus AHR gene. The predicted amino acid sequence contained regions characteristic of other vertebrate AHRs. However, in line with previously described fish AHR genes, no distinct Q-rich domain was found. Phylogenetic analysis demonstrated that Xenopus AHR was clustered within the AHR1 clade. As in the case of mammalian AHR genes, the Xenopus AHR gene was expressed in all the adult tissues tested. Xenopus AHR was also expressed during early development, in parallel with expression of the CYP1A7 gene, which is thought to be regulated by AHR. These results suggest that while frogs are relatively tolerant to TCDD toxicity, the AHR of frogs has characteristics similar to those of other vertebrate AHRs. (C) 2003 Elsevier Inc. All rights reserved.