The nuclear DNA content of cells from individual organs of seedlings and plants of Cucumis sativus L. (cv. Hokus) was determined during development by flow cytometry. The results revealed the presence of cells with different DNA contents (ranging from 2C to 64C) in varying proportions. This polysomaty was both specific to the developmental stage of the plant and to the individual organ. Polysomaty was already present in the hypocotyl and radicle of the embryo in the seed, and also in young secondary roots, immature leaves, flower buds and very young stem internodes. Rapid increases of the nuclear DNA contents (endopolyploidy) and the number of C-value levels occurred in all organs of the seedling and in the stem internodes and flowers during elongation. In the seedlings, these increases originated in the transition tissue between hypocotyl and radicle. In the leaves the initial pattern of polysomaty did not change during expansion. Compared to other organs, the leaves showed the highest frequency (60%) of cells with a 2C DNA content. During plant maturation, the proportion of nuclei with higher DNA contents gradually increased in the radicle, the hypocotyl and the stem internodes. The results are discussed in relation to organ elongation, to genome size and culture conditions, and to their relevance to molecular and cellular approaches for cultivar improvement.