The insulin-like growth factor type 1 receptor (IGF-IR) belongs to the tyrosine kinase (TK) receptor family. Besides being mitogenic, IGF-IR plays a crucial role in cell survival, transformation and maintenance of the malignant phenotype. In this study, we cloned the cDNA from the hypothalamus of the orange-spotted grouper (Epinephelus coioides) using reverse transcription PCR (RT-PCR) and the rapid amplification of cDNA ends (RACE) method. The deduced amino acid sequence showed that the receptor comprises 1413 amino acid residues. It contains cysteine-rich domains in its alpha-subunit, and a conserved transmembrane domain and TK domains in its beta-subunit. Comparison of the amino acid sequence with those of other species showed that the grouper IGF-IR shares 90.2%, 89.6%, 71.9% and 72% similarity with the IGF-IR of the Japanese flounder, turbot, zebrafish-a and zebrafish-b, respectively. When compared with its mammalian homologue, grouper IGF-IR contains a large insertion at its C-terminus. Phylogenetic analysis has revealed that the grouper IGF-IR belongs to the b-type IGF-IRs and has a higher similarity with flounder and turbot IGF-IR, and a lower similarity (<70%) with human, mouse and avian IGF-IR. Grouper IGF-IR transcripts were detected in the brain, peripheral tissues, embryos and early development larvae by semi-quantitative RT-PCR assay. It was observed that IGF-IR mRNA expression was greater in the brain than in peripheral tissues. The level of IGF-IR mRNA expression was much higher in retina, gonad, skeletal muscle and gill tissues than in liver, heart and thymus tissues. The expression of IGF-IR can be visualized as a ubiquitous signal in unfertilized eggs, embryos and early development larvae. The distribution pattern of IGF-IR mRNA in grouper development suggests that IGF-IR plays an important role in the embryo and early larval development stages.