Insulin is one of the most important endocrine hormones, playing a critical physiological role in regulating metabolism and glucostasis in fish. In this paper, the complete cDNA of largemouth bass (Micropterus salmoides) insulin gene was cloned by reverse transcriptionPCR (RT-PCR) and rapid amplification of cDNA ends (RACE) methods, and its encoding amino acid sequence as well as the protein structure was analyzed. The insulin fulllength cDNA sequence consists of 655 bp, comprising a 101 bp 5′untranslated region, a 213 bp 3′untranslated region and an open reading frame (ORF) of 351 bp in length. The ORF encodes a preproinsulin of 116 amino acids with a molecular weight of about 12.7 ku and a theoretical isoelectric point of 5.44, from which a signal peptide, a Bchain, a Cpeptide and an Achain could be derived. Online analysis suggests that the deduced preproinsulin contains a transmembrane region, and that it has a signal peptide whose cleavage site occurs between Alanine²⁴ and Phenylalanine²⁵. Comparative analysis of preproinsulin amino acid sequences related indicates that Achain and Bchain sequences of propreinsulins are highly conserved between teleost and other vertebrate species, and that the preproinsulin amino acid sequence of largemouth bass shares high similarity of 75%-94% to those of other teleosts, but low similarity of 59%-62% to those of other vertebrates. The phylogenetic tree by the Neighborjoining method shows that the preproinsulin of largemouth bass has high homology with that of some of the other Perciformes fishes, such as rock bass, greater amberjack and tilapia.