Cyclin-dependent kinases (CDKs) are a family of serine/threonine protein kinases which play a pivotal role in the eucaryote cell cycle regulation. We have identified the Xenopus homologue of mammalian CDK4 (XCDK4). The protein sequence of XCDK4 has 78 and 77% overall identity to human and mouse CDK4, respectively. Northern blot analysis revealed a single transcript of approximately 4.5 kb present at various stages. XCDK4 transcripts show very dynamic expression during early development. The level of expression is higher during cleavage and gastrulation. In situ hybridization analysis revealed that the transcripts are enriched in the dorsal mesoderm at the beginning of gastrulation, then extend to both the lateral and ventral mesoderm. At the end of gastrulation, XCDK4 transcripts are mainly distributed in the blastoporal region and in the anterior neural fold. During neurulation they become restricted to optic vesicles and to neural crest cells organizing the branchial arches. As development proceeds, XCDK4 transcripts are highly expressed in the branchial arches. At late tail-bud stages, XCDK4 transcripts are also detected in ventral hematopoietic precursor cells. Therefore, this analysis clearly shows that XCDK4 has a regionalized expression during Xenopus embryogenesis.