This study reports a factor analysis of the data resulting from a 3-year longitudinal investigation into conservation of number and related mathematical type concepts. Conservation of number, which represents a subset within the concept of conservation, was measured by use of a 20-item, criterion-referenced testing device. The subjects, who were tested 3 years in succession during a 4-week period each spring, were 5, 6, and 7 years of age (at the time of the last data collection). The factor analysis of the test results revealed a pattern of development that showed an increasing emergence of the conservation factor. Furthermore, successive factor analyses indicated that this conservation factor has its roots in related subskills, particularly vocabulary discrimination. The overall developmental pattern, as shown by this analysis, moves from diffuseness toward increasing organization as the subskills develop and conservation emerges. These results are seen as support for Piaget's theoretical notion of the emergency of conservation. (MH)