Understanding lunar phases requires three-dimensional information about the relative positions of the Moon, Earth, and Sun, thus using a stereo display in instruction might improve student comprehension of lunar phases or other topics in basic astronomy. We conducted a laboratory (15 sections) on phases of the Moon as part of the introductory astronomy classes. Half of the laboratories were taught using stereo visualizations projected by a portable GeoWall system running the AstroWall software, while the other half of the laboratories were identical, but without stereo. We found that both sets of laboratories showed a statistically significant gain in student comprehension, but that there was no statistical difference between the stereo laboratories and the nonstereo laboratories. We conclude that there is no advantage to using a stereo display in teaching about lunar phases. (Contains 3 tables and 3 figures.)