Two randomized control/treatment experiments tested the effectiveness of Chartworld software on enhancing mathematical knowledge of 3rd-graders. In Experiment 1, 196 children were randomly assigned to receive either Chartworld or textbook instruction on number operations. ANCOVAs indicated significant group differences in favor of Chartworld in the magnitude of posttest performance while controlling for pretested ability (p less than 0.05). For Experiment 2, which covered divisibility and prime numbers, the results demonstrated significantly greater gains in conceptual knowledge for Chartworld (p less than 0.001). These experiments provide corroborating evidence that Chartworld is an effective method of instructional delivery for conceptual knowledge. (Contains 7 figures and 2 tables.)