The cell expansion phase contributes in determining the major characteristics of a fleshy fruit and represents two-thirds of the total fruit development in tomato (Solanum lycopersicum). So far, it has received very little attention. To evaluate the interest of a genomic scale approach, we performed an initial sequencing of approximately 1,200 cell expansion stage-related sequence tags from tomato fruit at 8, 12, and 15 d post anthesis. Interestingly, up to approximately 35% of the expressed sequence tags showed no homology with available tomato expressed sequence tags and up to approximately 21% with any known gene. Microarrays spotted with expansion phase-related cDNAs and other fruit cDNAs involved in various developmental processes were used (1) to profile gene expression in developing fruit and other plant organs and (2) to compare two growing fruit tissues engaged mostly in cell division (exocarp) or in cell expansion (locular tissue surrounding the seeds). Reverse transcription-polymerase chain reaction analysis was further used to confirm microarray results and to specify expression profiles of selected genes (24) in various tissues from expanding fruit. The wide range of genes expressed in the exocarp is consistent with a protective function and with a high metabolic activity of this tissue. In addition, our data show that the expansion of locular cells is concomitant with the expression of genes controlling water flow, organic acid synthesis, sugar storage, and photosynthesis and suggest that hormones (auxin and gibberellin) regulate this process. The data presented provide a basis for tissue-specific analyses of gene function in growing tomato fruit.