The Sox family is a large group of transcription factors that are characterized by the presence of a DNA-binding HMG domain. We isolated HrSoxB1, an ascidian homolog of the Sox gene that belongs to the B1 subclass of the Sox family, from Halocynthia roretzi. Expression was initiated as early as the 8-cell stage. During cleavage stages, HrSoxB1 was expressed in three quarters of embryonic blastomeres but not in posterior-vegetal (B-line) blastomeres. Misexpression of mRNAs of HrPEM but not of macho-1, whose maternal mRNAs are localized to the posterior-vegetal cytoplasm of eggs and early embryos, repressed the anterior-vegetal expression of HrSoxB1. This result suggests that the zygotic expression of HrSoxB1 is controlled by the localized maternal mRNA. When HrSoxB1 was overexpressed in early embryos, ectopic expression of HrBra, a gene for a transcription factor expressed in notochord blastomeres, occurred in the most posterior blastomeres (B7.5), although these blastomeres did not eventually differentiate into notochord but developed into muscle, as they do in normal embryogenesis. In later embryogenesis, HrSoxB1 was specifically expressed in neural plate cells. However, overexpression of HrSoxB1 did not affect the expression of a neural plate marker gene, HrETR-1.