Notch signaling is involved in cell fate determination and is evolutionally highly conserved in vertebrates and invertebrates. Mastermind is a nuclear protein which participates in Notch signaling and is involved in direct transactivation of target genes. Here we analyzed the expression and the function of Xenopus mastermind1 (XMam1) in the process of primary neurogenesis. XMam1 is 3,425 bp and encodes 1,139 amino acids. Overall, Mastermind proteins consist of a basic domain, two acidic domains and a glutamine-rich domain, which are highly conserved among species. The ubiquitous expression of XMam1 was observed in both maternal and zygotic stages. Whole-mount in situ hybridization showed that XMam1 mRNA was present in the ectoderm by the gastrula stage and localized at the anterior neural region in the neurulla stage. Thereafter, XMam1 expression was restricted to the eye and otic vesicle in the tailbud-stage embryo. XMam1 overexpression caused the repression of primary neural formation. The truncated form of XMam1 (lacking the C-terminus of XMam1; XMam1DeltaC) led to excess formation of primary neurons. Furthermore, XMam1DeltaC strongly repressed XESR-1 transactivation. These results show that XMam1 is involved in primary neurogenesis by way of Notch signaling and is an essential component for transactivation of XESR-1 in Xenopus laevis embryos.