The aim of the present study is to explore the effects of Meta-strategic Knowledge (MSK) on scientific inquiry learning. MSK is a subcomponent of metacognition defined as general, explicit knowledge about thinking strategies. Following earlier studies that showed considerable effects of explicit instruction of MSK regarding the strategy of variables control, the present study explores whether similar effects are found in two additional scientific thinking strategies: Define Research Questions and Formulate Research Hypotheses. Participants were 119 eighth-grade students from six classes of a heterogeneous school. Equal numbers of low-achieving and high-achieving students were randomly assigned into experimental and control groups. The findings showed dramatic developments in students' performance following instruction. The effect of the treatment was preserved in a delayed transfer test. Our findings show that explicit teaching of MSK had a stronger effect for low-achieving students than for high-achieving students. The implications of the findings for teaching and learning in the context of scientific inquiry are discussed. (Contains 3 tables and 2 figures.)