In the brain, the conversion from androgen into estrogen is an important process for the differentiation of the brain function in male rodents. The aromatase is expressed in some nucleus of the brain. To assess the functional significance of the aromatase gene in development and activation of sex-specific behavior, we analyzed behavioral phenotypes of the aromatase knockout (ArKO) male mice. ArKO males obviously decreased their fertility and showed deficits in male sexual behavior including mount, intromission and ejaculation. Noncontact penile erection was not significantly affected by defect of the aromatase gene. A reduction of aggressive behavior against male intruders was also observed in ArKO males, while they tend to exhibit aggression toward estrous females during male copulatory tests. Moreover, the infanticide toward the pups was observed in the ArKO males, whereas characteristic parental behavior, but not infanticide was observed in wild-type males. These results indicate that aromatase gene expression is a critical step not only for motivational and consummatory aspects of male sexual behavior, but also for aggressive and parental behaviors in male mice.