We cloned the chick homologue of Homo sapiens thymosin beta4, encoding a G-actin sequestering factor which plays an important role in angiogenesis, cell motility and tumorigenesis. The thymosin beta4 gene is highly conserved between chick and human. Its expression was analyzed during different stages of development. At early stages thymosin beta4 is expressed in the mesoderm and endoderm and in Hensen's node. Later, thymosin beta4 transcripts are found in the head mesenchyme, somites, dorsal root ganglia, neural tube, brain, blood vessels and feather buds. The pattern of thymosin beta4 expression in blood vessels indicates a function mainly in development of the blood circulatory system which closely parallels findings in vitro. The observed expression pattern shows a high similarity to expression data published for mice, mainly in the heart and in the nervous system. Important new aspects are the early onset of expression, the expression in the mesoderm preceding heart formation and the involvement in feather development.