A nifU- like gene exhibiting similarity to nifU of nitrogen fixation gene cluster was identified for the first time from the gut cDNA library of amphioxus Branchiostoma belcheri. Both RT- PCR and Northern blotting as well as in situ hybridization histochemistry verified that the cDNA represents an amphioxus nifU- like gene rather than a microbial contaminant. The nifUlike gene encodes a protein of 164 amino acid residues including a highly- conserved U- type motif ( C-X-26 -C-X-43 -C), and shares 66 - 86% identity to NifU- like proteins from a variety of species including vertebrates, invertebrates and microbes. It is expressed in a tissue- specific manner in the digestive system including epipharyngeal groove, endostyle, hepatic caecum and hindgut and in the gill, ovary and testis. Taken together, it is highly likely that NifU- like protein plays some tissue- dependent and critical role in amphioxus.