The effects of the estrogenic compound 4-nonylphenol (4-NP) on vitellogenesis in juvenile olive flounder ($Paralichthys$ $olivaceus$) and rockfish ($Sebastes$ $schlegeli$) exposed continuously at 10, 50 and $100{mu}g{ell}^{-1}$ levels for 7 days were compared. The expression of VTG mRNA level and protein using specific probes were examined. The levels of plasma estradiol-17 ${eta}(E_2)$ and testosterone (T) were assessed by radioimmunoassay (RIA). Plasma $E_2$ concentrations increased significantly in two female fish species exposed to $100{mu}g{ell}^{-1}$ of 4-NP over concentrations in control fish. Plasma T concentrations increased in $P.$ $olivaceus$. Four days after exposure, the level of VTG mRNA expression increased in $P.$ $olivaceus$ and $S.$ $schlegeli$ exposed to $20{mu}g{ell}^{-1}$ of 4-NP. In addition, plasma VTG protein expression was seen in $P.$ $olivaceus$ and $S.$ $schlegeli$. In $S.$ $schlegeli$ and $P.$ $olivaceus$ exposed to 4-NP, the changes were noticed mainly in hepatocytic vaculation after 7 days of exposure. Thus, 4-NP may disrupt vitellogenesis in immature fish both directly and indirectly via disrupted steroidogenesis and liver pathology. Immature $S.$ $schlegeli$ were the most sensitive to 4-NP exposure in vitellogenesis.