This study assessed the effects of explicit teaching of metastrategic knowledge (MSK) on gains of low-achieving (LA) and high-achieving (HA) 5th grade students (N=41). Gains in reasoning scores of students from the Experimental group (compared to students from the control group) were obtained on the strategic and on the metastrategic level. Gains were preserved in near and far transfer tasks immediately after the end of instruction and 3 months later. Explicit teaching of MSK affected both LA and HA students, but it was extremely valuable for LA students who required a longer period than HA students to reach their top score. (Contains 3 tables and 4 figures.)