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The effects of a dynamic instructional environment (based on use qfThe Geometer's Sketchpad, N. Jackiw, 1991, in a computer lab) and visualization on eighth-grade students9 fN = 241) construction of the concepts of reflection and rotation were investigated. Also investigated were the effects of the environment on students' two-and three-dimensional visualisation. After controlling for initial differences, it was concluded that students experiencing the dynamic environment significantly outperformed students experiencing a traditional environment on content measures of the concepts of reflection and rotation, as well as on measures of two-dimensional visualization. The students' environment did not significantly affect their three-dimensional visualization. The Curriculum and Evaluation Standards for School Mathematics (National Council of Teachers of Mathematics [NCTM], 1989) include the following objectives in the standard for middle-level geometry: "In grades 5-8, the mathematics curriculum should include the study ofthe geometry of one, two, and three dimensions in a variety of situations so that students can visualize and represent geometric figures with special attention to developing spatial sense; [and] explore transformations ofgeometric figures" (p. 112). This paper demonstrates that this standard is not being adequately implemented with all students and provides a strategy for its achievement.The Second International Mathematics Study: Report fertile United States (Crosswhite, 1985) identified topics taught in other countries that were not covered by the majority of eighth-grade classes in the United States. "Forexample, topics in transformational geometry, taught in some countries, were reportedly taught by only 12% of United States eighth grade teachers" (p. 20). Furthermore, results ofthe study indicated that teachers primarily taught according to their textbook, with little use of manipulatives or other materials not included with the text. This finding was corroborated by data collected in a comparative study with seventh-and eighth-grade students in the United States and Japan (Iben, 1988).

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