Using the in vivo male medaka model, we investigated the estrogen‐like response of perfluorooctyl iodide (PFOI) which is a potential source of perfluorinated carboxylic acids. Using real‐time quantitative polymerase chain reaction, the expression levels of related estrogenic genes including estrogenic receptor α (ERα), ERβ, vitellogenin I (VTG I), and VTG II in the livers of male medaka exposed to PFOI were analyzed. The results showed that PFOI upregulated the expression levels of the tested genes in a dose‐dependent manner. VTG protein levels increased in both dose‐ and time‐dependent manners due to PFOI exposure. The results suggested that PFOI is a potential estrogenic compound. © 2011 Wiley Periodicals, Inc. Environ Toxicol 28: 571–578, 2013.