We cloned cDNAs coding for the Xenopus counterparts of a type I activin receptor. The cDNA clones were predicted to encode 56kd proteins, namely XAR3 and XAR4 proteins. The two proteins are highly homologous to each other, showing 96% identity in the amino acid sequences. The GST-fused kinase domain of XAR3 autophosphorylated itself in vitro on threonine residues. The expression of XAR4 was found throughout embryogenesis, from oocytes to tailbud embryos, and also in adult tissues. By whole mount in situ hybridization, the XAR4 transcripts were detected in the animal half of blastulae and dorsally in gastrulae and neurulae. In tadpoles, the transcript was seen in the brain and around the otic vesicles. These results show that the activin type I receptor is expressed during Xenopus embryogenesis and suggest that the type I and II receptors are expressed together both temporally and spatially, supporting the idea that activin induces mesoderm in the embryo through activation of the two types of receptors.