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Agnathans, represented by lamprey and hagfish, are believed to be the oldest vertebrates, currently proved to possess the adaptive immune defenses [1,2]. Though T-cell receptor (TCR) and B-cell receptor do not exist in jawless vertebrates, recent findings in lamprey have revealed that it possesses an alteative immune system that could specifically recognize and respond to exteal pathogens [1]. This system undergoes germline genomic rearrangements of insertion of diverse leucine-rich repeat modules to generate a large number of different variable lymphocyte receptors (VLRs) for resisting the invasion of pathogens [1,2]. Three types of receptors, VLRA, VLRB, and VLRC, have been identified in lampreys [3]. Recent evidence indicated that VLRA and VLRB are expressed in different cell types that resemble T and B cells of jawed vertebrates, respectively. After being infected by specific pathogen, VLRB lymphocytes express specific VLRB molecules which undergo amplified expression and begin to secrete VLRB in a manner analogous to the secretion of immunoglobulin by B cells in jawed vertebrates [3]. Characteristic genes expressed by VLRA, VLRB, and VLRC lymphocyte populations include transcription factors, cytokines,or che-mokines and their receptors, integrins, toll-like receptors (TLRs), and various signaling molecules [1,3]. Metamorphosis is present in all families of Petromyzoniformes, the lampreys. During lamprey metamorphosis, dramatic biochemical, cellular, and morphological changes occur at the time of lamprey metamorphosis that are crucial for this event, as well as for the maintenance of larval behavior and metabolism [4]. Gene expressions of VLRA, VLRB, and VLRC lymphocytes are of interest before and after metamorphosis, which may offer new insights into the evolution of adaptive immunity. Comparison of the expressions of a selective panel of genes between larva and adult will reveal different gene expression profiles, and the distinctions are particularly informative for the larva and adult. Here, we described the gene expressions of VLRA, VLRB, and VLRC lymphocytes before and after metamorphosis.