Thymosin beta 4, a polypeptide of 5 kDa, is known to have capacity to regulate actin polymerization by binding to an actin monomer. Distribution of Xenopus laevis thymosin beta 4 (XT beta 4) in the developing Xenopus larva was examined by means of in situ hybridization and immunohistochemistry. Analysis with in situ hybridization revealed that XT beta 4 mRNA becomes gradually localized to the neural tissues, notochord and inner epidermis during neurula stages. Intense accumulation of XT beta 4 mRNA was observed in the ganglions of cranial nerves and in the dorsal region of the spinal cord from stage 26 and onwards. XT beta 4 immunoreactivity (XTI) was observed in larvas at all developmental stages later than stage 26, tail bud embryo. Immunoreactivity was initially distributed to the ganglion of cranial nerve V and Rohon-Beard cells. As the development progressed, the XTI appeared in other neuronal groups. By late tadpole stages (stages 42-47) the XTI was found in the pineal body, oculomotor and trochlear motoneurons of the midbrain, various neurons in the rhombencephalon, ganglions of cranial nerves V, VII/VIII and IX/X. In the spinal cord the XTI was observed in Rohon-Beard cells, dorsal root ganglion cells, motoneurons and other spinal cord neurons. Immunoreactivity was seen in both cell bodies and axons of the neurons. These findings suggest that thymosin beta 4 plays a role in the development of neurons, especially of sensory neurons.