In the statistical and transcriptional analyses reported in this exploratory study, original rhythms of 6-11-year-old children (N=36) were examined. The hypotheses were based on a new model of musical development, and tested empirically using non-pitch rhythmic improvisation in a MIDI-environment. Several representational types were found in cluster analysis.The types are comparable to those found in earlier studies of rhythm. A dimensional conflict appeared between grouping and metre, and was solved in the last substage (10-11 years). Age was found to be a significant factor in the development of the hierarchy of form, the amount of different rhythm patterns, the metrical hierarchy, as well as the understanding of the relation of grouping and metre in rhythmic improvisation. The results supported the hypotheses that, in the first substage, children focused on either the surface (rhythmic figures) or deep (metric hierarchy) structures of the event. In the next substage, surface and deep structures started to become coordinated, and by the last substage, they were fully integrated. (Contains 12 figures, 1 table, and 2 notes.)