Direct teaching of problem-solving methods to high school physics students met with little success. Expert problem solving depended upon an organized knowledge base. Concept mapping was found to be a key to organizing an effective knowledge base. The investigation of the effect of the degree of concept mapping on achievement was the purpose of this study. Six intact high school physics classes, taught by this investigator, took part in the study. Two classes were control groups and received standard instruction. Four classes received six weeks of concept-mapping instruction prior to the unit under study. Two of these four classes were the low-level treatment group and were required to submit concept maps at the conclusion of the instruction. The other two classes were the high-level treatment group and were required to submit concept maps at the beginning and at the conclusion of the unit under study. One class from each treatment group took a pretest prior to instruction. An analysis of the posttest results revealed no pretest sensitization. A one-way analysis of covariance indicated a significant main effect for the treatment level at the p < 0.05 level. A pair of single-df comparisons of the adjusted treatment means resulted in significant differences (p < 0.05) between the control group and the average of the treatment means as well as between the two experimental groups. It can be concluded that for this sample (upper-middle-class high school physics students) mapping concepts prior to, during, and subsequent to instruction led to greater achievement as measured by posttest scores.