The current goal of our study is understanding of molecular mechanism of trunk and tail development, especially somite development. For this purpose, we have performed ENU mutagenesis screening for mutations affecting morphogenesis of somites in zebrafish and expression pattern screening for genes expressing in the psesomitic mesoderm and the tail bud. With regard to the mutagenesis screening, we obtained 15 mutants with defects in the formation of epithelial somites and mutants with defects in the formation of the tail bud,in 615 F2 families, which corresponded to 662.2 mutagenized genomes. On the other hand, we identified at least 50 genes specifically expressed in the PSM and the tailbud by an in situ hybridization screening of mRNA expressed in the PSM to tailbud region of zebrafish embryos (18- to 21-somite stages).