In teaching inquiry to high school students, educators differ on which method of teaching inquiry is more effective: Guided or open inquiry? This paper examines the influence of these two different inquiry learning approaches on the attitudes of Israeli high school biology students toward their inquiry project. The results showed significant differences between the two groups: Open inquiry students were more satisfied and felt they gained benefits from implementing the project to a greater extent than guided inquiry students. On the other hand, regarding documentation throughout the project, guided inquiry students believed that they conducted more documentation, as compared to their open inquiry peers. No significant differences were found regarding "the investment of time", but significant differences were found in the time invested and difficulties which arose concerning the different stages of the inquiry process: Open inquiry students believed they spent more time in the first stages of the project, while guided inquiry students believed they spent more time in writing the final paper. In addition, other differences were found: Open inquiry students felt more involved in their project, and felt a greater sense of cooperation with others, in comparison to guided inquiry students. These findings may help teachers who hesitate to teach open inquiry to implement this method of inquiry; or at least provide their students with the opportunity to be more involved in inquiry projects, and ultimately provide their students with more autonomy, high-order thinking, and a deeper understanding in performing science.