Studied was how two different methods of employing instructional materials in teaching elementary mathematics related to students' achievement in basic mathematical understandings. The sample consisted of fourth grade students in a university laboratory school. Students were randomly assigned to two groups, with an equivalent number of boys and girls in each group. The same lesson was taught to each group, with the presentation differing only in the manner in which the instructional materials were utilized. The control group saw only a teacher demonstration of the materials while the experimental group was given the materials to manipulate individually. Among the findings were (1) no significant difference between class means as determined by tests for understanding of basic mathematical principles and general mathematical achievement, (2) the group using individually manipulated materials made greater gains in proficiency than the group seeing only a teacher demonstration, (3) the boys in the experimental group made greater mean gains than the boys in the control group, and (4) the girls in the experimental group made a greater mean gain than the boys in an instrument designed to measure understanding of basic mathematical principles. (RP)