Marine water is contaminated with polycyclic aromatic hydrocarbons (PAHs) from rivers and the atmosphere Oil spilled from tankers or petroleum factories also causes marine pollution with PAHs It is well known that PAHs, such as benzo[a]pyrene, show carcinogenicity and/or mutagenicity In developing teleost fish, furthermore, it has been reported that spinal deformity was induced in Pacific herring and pink salmon by PAHs, although the detail mechanism of toxicity in teleosts is not elucidated yet As a toxic pathway of PAHs in animals, the metabolic activation of PAHs in the presence of P450 is considered In teleosts, as well as in mammals, it has been reported that PAHs are converted into monohydroxylated polycyclic aromatic hydrocarbons (OHPAHs) by an enzyme cytochrome P4501A1 Thus, OHPAH might have a toxic effect in teleosts In the present study, we examined the estrogenic activity of OHPAHs using the yeast-two hybrid assay system with human estrogen receptor (ER) a As a result, we detected estrogenic activity in 4-hydroxybenz[a]anthracene (OHBaA) Then, the direct effect of 4-OHBaA on osteoclasts and osteoblasts in teleosts was examined using the assay system with fish scales, which contain osteoclasts, osteoblasts, and the bone matrix of two layers (bony layer a thin, well-calcified external layer, a fibrillary layer a thick, partially calcified layer) When the scales of goldfish and wrasse were incubated with 4-OHBaA, which showed agonistic activity in the yeast-two hybrid assay system described above, the suppressing effect was observed for both osteoclasts and osteoblasts The osteoclastic and osteoblastic activities did not change with 1-hydroxypyrene, which has no binding activity to human ER alpha Thus, we conclude that PAH metabolites have a toxic effect on fish bone metabolism and that careful attention should be given to aquatic PAH contamination