Phoenix dactylifera (date palm) is a dioecious species displaying strong dimorphism between pistillate and staminate flowers. The mechanisms involved in the development of unisexual flowers are as yet unknown.This paper describes the results of inflorescence and flower development studies using different histological and molecular cytological approaches. Nuclear integrity and cell division patterns in reproductive organs were investigated through DAPI staining and in situ hybridization using a histone H4 gene probe.The earliest sex-related difference in flower buds is observed at an otherwise 'bisexual' stage, at which the number of cells in the gynoecium of pistillate flowers is higher than in their staminate counterparts. In the pistillate flower, staminodes (sterile stamens) display precocious arrest of development followed by cell differentiation. In the staminate flower, pistillodes (sterile gynoecium) undergo some degree of differentiation and their development ceases shortly after the ovule has been initiated. Staminode and pistillode cells exhibit nuclear integrity although they did not show any accumulation of histone H4 gene transcripts.These results strongly suggest that the developmental arrest of sterile sex organs and the subsequent unisexuality of date palm flowers result from a cessation of cell division and precocious cell differentiation rather than from cell death.