Protein phosphorylation and dephosphorylation are the most important regulatory mechanisms governing many aspects of biology. Phosphoproteome studies have revealed that phosphorylation and dephosphorylation modulate functions of more than one third of the total cellular proteins. In eukaryotes, dephosphorylation at serine/threonine residues are executed by four major protein phosphatases, Phosphatase-1(PP-1), Phosphatase-2A(PP-2A), Phosphatase-2B(PP-2B), Phosphatase-2C(PP-2C), and several minor phosphatases including Phosphatase-4(PP-4), Phosphatase-5(PP-5), Phosphatase-6(PP-6), and Phosphatase-7(PP-7). Among these different phosphatases, Protein Phosphatase-1(PP-1) is one of the most important protein serine/threonine and plays distinct roles in regulating gene expression, signal transduction, cell proliferation, differentiation, transmission, apoptosis, autophagy, morphogenesis, organogenesis and other cellular activities. Our previous studies have demonstrated that PP-1 is a major phosphatase that dephosphorylates Pax6 to modulate its function in regulating brain and eye development. More recently, we have established the expression patterns of the catalytic subunits and the regulatory subunits for PP-1 in mouse eye. To explore the possible functions of PP-1 in various tissues of the lower vertebrates, here, we have analyzed the differential expression patterns and the cellular localizations of the catalytic subunit for PP-1 using western blot and immunohistochemistry on four different ploidy fish: the allotetraploid hybrids and their diploid parents, common carp (♂) and red crucian carp (♀) as well as the triploid crucian carp derived from crossover between the allotetraploids and common carp or between the allotetraploids and the red crucian carp. Our study demonstrated the following results: (1) PP-1c is expressed in the brain, heart, muscle, kidney, liver, and gonads with defined differential expression patterns. (2) The most striking feature is that a relatively higher level of PP-1c expression was found in the muscle of the above fish. (3) Compared among the muscle tissues from 4 types of fish, the lowest level of PP-1c expression was detected in the allotetraploid fish, an intermediate level of PP-1c detected in both parents and the highest level of PP1c observed in the triploid crucian carp. Such a pattern illustrates its variability between filial generation and the corresponding parents. (4) The immunohistochemistry study revealed similar localization of PP-1c in certain groups of cells within the same tissue from the four different organisms. Together, these results lead to the following conclusions: (1) PP-1c is differentially expressed in various tissues of the four different ploidy level fishes; (2) PP-1c functions are highly regulated in different tissues; (3) Within the same tissue of the four types of fish, PP-1c is localized in the same types of cells; and (4) the presence of strong PP-1c immunofluorescence signal in certain nuclei of neurons in both allotetraploid and triploid brains but not in those of the diploid parents indicates that PP-1c may be used as a biochemical marker to distinguish the different types of fish. In summary, our results represent the first report on the differential expression patterns of the protein phosphatase-1c in six tissues from the different ploidy level fish, and provide valuable information for the future study of the PP-1c functions in these organisms.