The paper describes characteristics and analyses difficulties of environmental sciences teachers in mentoring their students in an extended inquiry project, which is a mandatory requirement of the environmental sciences matriculation in Israel. The teachers participated in a professional development program that provided both content knowledge and support for conducting inquiry, and enabled the teachers to guide the students in the inquiry project. Teachers who had experience in inquiry identified more skills required for mentoring students' inquiry, and provided a non-directive guidance pattern, whereas inexperienced teachers acknowledged less skills, and tended to present a directive-authoritative approach. Insufficient content and pedagogical content knowledge affected the teachers who closely controlled their students' work.