We used comparative two-dimensional gel electrophoresis to highlight proteins that are differentially expressed in the maturation stage of the parasite Opisthorchis viverrini (OV). The proteins differentially expressed in the juvenile/adult forms of the parasite are thought to be important for survival and pathogenesis. We used a nonlinear gradient pH ranged 3-10 strips for isoelectric focusing to resolve soluble proteins from four different maturation periods of OV from 1 week juvenile to 4 week adult. Approximately 210-240 protein spots were resolved by 2-DE in two ranges of pI (4.5-5.8 and 6.0-8.0). At least 35 protein spots were differentially expressed in 4 week adult compared to 1 week juvenile fluke. These proteins may involve in sex organ development and egg production. Comparative analysis of the OV proteome of different aged parasites during maturation may help to better understand parasite biology, pathogenesis/carcinogenesis related to this parasite and lead to the identification of new targets of vaccines and drugs.