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Fruit position on the bunch (in¯orescence) is an important part of variability in banana fruit weight at harvest, as fruits at the bottom of the bunch (distal fruits) are approx. 40 % smaller than those at the top ( proximal fruits). In this study, the respective roles of cell number and cell ®lling rate in the development of pulp dry weight are estimated. To this end, the source/sink ratio in the plant was altered at dierent stages of fruit development. Leaf shading (reducing resource availability), bunch bagging (increasing sink activity by increasing fruit temperature), and bunch trimming (decreasing sink size by fruit pruning), applied once cell division had ®nished, showed that the pulp ®lling rate depends on resource availability. Bunch bagging and bunch trimming were also carried out before the end of cell division to estimate the role of pulp cell number in the development of pulp dry weight. A sampling method was calibrated to evaluate pulp cell number from the digestion of a ®xed portion of the pulp in a solution of chromic and nitric acids. A relationship was found between pulp cell number and fruit length at the end of cell division. It was observed that pulp cell number is a determining factor in pulp dry weight variability within a bunch. On the other hand, the cell ®lling rate was identical for all fruits in the bunch and was in¯uenced by the source/sink ratio. A Michaelis-Menten relationship was invoked to relate the cell ®lling rate in a bunch to the source/sink ratio during bunch ®lling.# 2001 Annals of Botany Company