In this study, we investigated how Grade 3 and 4 students' organizational structure for volume units develops through repeated experiences with a virtual manipulative for building prisms. Our data consist of taped clinical interviews within a micro-genetic experiment. We report on student strategy development using a virtual manipulative for counting cubes as a measure of prism volume. A descriptive case of one student, Jim, is included as an example of how students developed increasingly efficient counting strategies built on understanding of structuring, composite units, multiplicative thinking and an understanding of the number of cubes along an edge. We found students were able to develop structure for volume and advance their level of thinking along a learning trajectory for volume measure. [For complete proceedings, see ED581294.]