In this study a two-sample, pre/posttest, quasi-experimental design was used to investigate the effect of explicit problem-solving instruction on high school students' conceptual understanding of physics. Eight physics classes, with a total of 145 students, were randomly assigned to either a treatment or comparison group. The four treatment classes were taught how to use an explicit problem-solving strategy, while the four comparison classes were taught how to use a textbook problem-solving strategy. Students' problem-solving performance and conceptual understanding were assessed both before and after instruction. The results indicated that the explicit strategy improved the quality and completeness of students' physics representations more than the textbook strategy, but there was no difference between the two strategies on match of equations with representations, organization, or mathematical execution. In terms of conceptual understanding, there was no overall difference between the two groups; however, there was a significant interaction between the sex of the students and group. The explicit strategy appeared to benefit female students, while the textbook strategy appeared to benefit male students. The implications of these results for physics instruction are discussed.