Low participation rates of girls in advanced physics classes are a serious problem in many countries. Assuming that physics teachers can affect girls' choice to elect advanced physics classes, and that teachers' behaviors are affected by their knowledge and beliefs, the goal of this study was to investigate physics' teachers' knowledge and beliefs regarding girls' low participation rates in physics. Interviews were conducted with 25 physics teacher who taught in 25 different high schools in an urban area in Israel. The findings show that: approximately one-half of the teachers underestimate the severity of the problem; almost two-thirds of the teachers do not see it as a problem that requires any action; and most teachers do not know what can be done to encourage girls to elect physics and to create a more gender inclusive physics learning environment. The implications for physics teacher education are discussed.