This study examined the effectiveness of worksheets while learning about biodiversity in a natural history museum. Despite the frequent use of worksheets by school classes during out-of-school activities, their effectiveness in enhancing knowledge acquisition has been addressed by relatively few empirical studies. 148 Austrian grammar school students aged eleven to fifteen took part in the pre- and post-test questionnaire study which included a one-hour learning phase with worksheets in the museum. Results indicate a high learning effect from pre- to post-test. Further analyses show that worksheets contributed to knowledge gain, but this contribution is similar to that afforded by prior knowledge. The design of the worksheet tasks was also important. A closed task setting required the students to look for clearly defined solutions. This led to greater knowledge gains than open worksheet tasks, where the spectrum of possible answers was wider. We concluded that worksheets can be used effectively for acquiring basic knowledge about biodiversity and suggest how to integrate the findings into the design of worksheets for out-of-school learning. (Contains 2 figures and 2 tables.)