Insulin-like growth factor I (IGF-I) is an important regulator of growth and development in vertebrates, and its biological actions are by binding to membrane-specific IGF-I receptors (IGF-IRs). Therefore, the analysis of expression can help to further elucidate the role for IGF in the regulation of developmental processes. In this study, the expression patterns of and its receptor genes during the embryonic development of Japanese flounder () were analyzed by fluorescent real-time PCR using embryos at different developmental stages. We observed that maternal transcripts of were present in early embryos and that their expression is developmentally regulated at different embryonic stages. A few transcripts are gradually increased from the eye lens formation stage to the pre-hatching stage. exhibit distinct temporal expression patterns during embryonic development. , and these appear at the transcripts are abundantly expressed at the gastrula stages and maintain a high level at the neurula, By contrast, the considerable transcripts of transcripts decrease significantly following . This indicates that the two IGF-I receptors might play different functional roles during