To evaluate the effects of benzo[a]pyrene (B[a]P), one of polycyclic aromatic hydrocarbons (PAHs), on in vitro oocyte maturation (GVBD) and sex steroid hormone production, maturing oocytes (oocyte diameters=0.74, 0.88 and 0.93 mm) of the longchin goby, Chasmichthys dolichognathus were incubated with B[a]P (1, 10 and 100 ng/mL) for 24 hours. After incubation, the oocytes were fixed with clearing solution (ethanol:formalin:glacial acrtic acid=6:3:1). The location of the germinal vesicle was observed under low-power magnification using a dissecting microscope. Steroids in aliquots of the incubation media were extracted twice using five volumes of ethylacetate:cyclohexane (1:1). Then, the T, E2 and 17α20βP levels were measured by RIA. In oocytes 0.74 mm diameter (vitellogenic oocytes), B[a]P had no significant effect on GVBD at the concentrations tested. In oocytes 0.88 mm diameter (fully vitellogenic oocytes), B[a]P inhibited GVBD significantly at 1 and 100 ng/mL. T production was decreased and the ratio of E2/T was increased significantly at 1 and 10 ng/mL compared with control. In 0.93 mm diameter oocytes (germinal vesicle located near the center of oocytes), B[a]P induced GVBD significantly at 10 and 100 ng/mL and decreased the ratio of E2/T significantly at 1 and 10 ng/mL compared with control. These findings suggest that B[a]P has different sensitivity to the oocyte maturation according to the oocyte diameters.