Cytochrome P450 (CYP1A) gene expression in the liver and sex steroid levels in plasma were investigated in olive flounder (Paralichthys olivaceus) exposed to tributyltin (TBT) and benzo[a]pyrene (BaP). We constructed a cDNA library and cloned a 230-base sequence encoding partial CYP1A DNA. The CYP1A gene expression level was estimated using northern blotting. Hepatic CYP1A mRNA levels in fish injected with BaP at 10 mg/kg body weight (b.w.) increased for 48 h after injection. However, fish injected with both BaP and TBT at 10 mg/kg b.w. showed no significant changes in CYP1A mRNA level after 48 h. Plasma concentrations of testosterone and $17{eta}$-estradiol were not significantly different in males and females injected with BaP and TBT. We suggest that TBT-induced suppression of BaP bioactivity should be interpreted with caution in biomonitoring field studies.