Primordial germ cells (PGCs) are the ancestor cells of adult gametes at embryonic stage. PGCs-specific gene expression plays a fundamental role in germ cell induction, development, migration and differentiation. Understanding the functional role of PGCs-specifically expressed genes could provide us some tools to perform gene and embryo manipulations. Screening a public database of zebrafish (http://www.zfin.org) we found a PGCs-specific candidate gene, carbonic anhydrase 15b (ca15b). To confirm this, whole-mount in situ hybridization and other technologies were utilized to analyze the expression of ca15b during zebrafish early development. We observed that ca15b is a maternally expressed gene and it is highly expressed in germ plasm during cleavage stage. The expression of ca15b at mRNA level was specifically detected in the PGCs at blastula stage. At gastrula stage, ca15b is still strictly expressed in PGCs while its expression is undetectable in somatic cells. This specific expression pattern of ca15b persisted until one day post-fertilization. More importantly, the post-transcriptional regulation elements that mediate PGCs-specific expression of ca15b may locate in its coding sequence.