The aim of this study was to characterise changes in leaf shape prior to phyllode acquisition along the axes of Acacia mangium seedlings. The study area was located in North Lampung (South Sumatra, Indonesia), where these trees belong to a naturally regenerated stand. A total of 173 seedlings, less than three months old, were described node by node. Leaf shape and leaf length were recorded and the way in which one leaf type succeeded another was modelled using a hidden semi-Markov chain composed of seven states. The phyllotactical pattern was studied using another sample of forty 6-month-old seedlings. The results indicate (i) the existence of successive zones characterised by one or a combination of leaf types, and (ii) that phyllode acquisition seems to be accompanied by a change in the phyllotactical pattern. The concepts of juvenility and heteroblasty, as well as potential applications for taxonomy are discussed.