Members of the Sox family of transcription factors are involved in a number of crucial developmental processes, including sex determination, neurogenesis and skeletal development. LSox5 is a member of the group D Sox factors that, in conjunction with Sox6 and Sox9, promotes chondrogenesis by activating the expression of cartilage-specific extracellular matrix molecules. We have cloned the chicken homologue of LSox5 and found that it is initially expressed in the premigratory and migratory neural crest after Slug and FoxD3. Subsequently, the expression of LSox5 is maintained in cephalic crest derivatives, and it appears to be required for the development of the glial lineage, the Schwann cells and satellite glia in cranial ganglia. Misexpression of LSox5 in the cephalic neural tube activated RhoB expression throughout the dorsoventral axis. Furthermore, the prolonged forced expression of LSox5 enlarged the dorsal territory in which the neural crest is generated, extended the 'temporal window' of neural crest segregation, and led to an overproduction of neural crest cells in cephalic regions. In addition to HNK-1, the additional neural crest cells expressed putative upstream markers (Slug, FoxD3) indicating that a regulatory feedback mechanism may operate during neural crest generation. Thus, our data show that in addition to the SoxE genes (Sox9 and Sox10) a SoxD gene (Sox5) also participates in neural crest development and that a cooperative interaction may operate during neural crest generation, as seen during the formation of cartilage.