G protein-coupled receptors (GPCR) constitute the largest superfamily of cell membrane receptors, mediating diverse signal-transduction pathways. The melanocortin 4 receptor (MC4R) has been of interest for its physiological role and size, one of the smallest among the GPCRs, which makes it a good model system for the structural study of GPCRs. To study the molecular structure and tissue-specific expression of MC4R in olive flounder (Paralichthys olivaceus), the full-length MC4R gene was obtained using PCR amplification of genomic DNA as well as cDNA synthesis. Sequence analysis of the gene indicates that 978 bp of the MC4R gene encodes 325 amino acids without introns. Sequence alignment with the MC4Rs from other fish shows the highest degree of identity (96%) between Paralichthys olivaceous and Verasper moseri, followed by Takifugu rubripes and Tetraodon nigroviridis (89%). RNA was isolated from various tissues to examine the tissue distribution of MC4R by using RT-PCR. The results showed major expression of MC4R in the liver, brain, and eye, which is consistent with the expression pattern in other fish belonging to the order Pleuronectiformes.