Low-achieving seventh-grade students from an urban parochial school were randomly assigned to two equally sized groups (n = 62, each group). One group was taught by a read-and-discuss, teacher-directed method, and the second group, given the same type of introductory lesson as the first, followed a model of concept mapping that connected major and minor concept ideas. A criterion-referenced test based on the content of a science chapter served as the dependent variable. Prior to any teaching, a pretest was administered. An analysis of covariance with pretest scores as the covariate showed a statistically significant difference in comprehension between the pretest and posttest for the experimental group. Effect size estimates revealed that concept mapping can be expected to improve comprehension scores of low-achieving seventh graders by approximately six standard deviations over a traditional instructional technique. When students lack background information on a topic to aid comprehension, the active participation in constructing semantic or concept maps may help students form a cognitive schema to assimilate and relate the new topic information.