Abstract: The reversible phosphorylation of proteins is an important posttranslational modification in eukaryotes that modulates the functional status of more than thirty percent of total cellular proteins. In the present study, we reported the molecular cloning of a partial cDNA coding for the PR55/B of PP-2A from the brain of goldfish through 5 RACE PCR strategy. The partial PR55 cDNA contained 1218 nucleotides which encoded a deduced partial protein of 405 amino acids. Sequence homology analysis showed that the PR55/B of PP-2A displayed a high level of amino acid identity with the counterpart from other species including human and rat, indicating the conservation of PR55/B. RT-PCR analysis revealed that PR55/B mRNA was specifically expressed in the brain and fin of goldfish. Our demonstration that PR55/B was expressed in the fish fin was a novel finding for the first time. This result suggested that the PP-2A with PR55/B as the regulatory subunit in fish likely played an important role in swimming, balancing and sensitivity to the water environment. Moreover, during the development of goldfish, PR55/B mRNA was initially detected at neurula stage, suggesting that the PP-2A with PR55/B as the regulatory subunit was likely implicated in control of the brain development differentiation in fish. Furthermore, we found that PR55/B became gradually increased from the optic vesicle stage and reached a peak level at the muscle movement stage, and thereafter, PR55/B mRNA maintained at this level with slight fluctuation from heart beat to hatching larvae. These results indicated that PP-2A with PR55/B as the regulatory subunit was actively regulating the development processes of these different stages. The consistent develop-mental expression patterns of PR55/B (Fig. 2) and the catalytic subunits (PP-2Ac)28 also supported that PR55/B may regulate fish development in holoenzyme, which was different from that of certain regulatory subunits of PP-2A20―22. Thus, our present study demonstrated that the specific PP-2A with PR55/B as regulatory subunit may play an important role in regulating fish development. In addition, PP-2A containing PR55/B as the regulatory subunit may play impor-tant roles of homeostasis in brain and fin.