Ultrastructural features of the compatible pollen tube ofPrunus avium(L.) growingin vivoare described. During its growth the tube has four different zones reflecting a functional specialization. The cytoplasm of the apical or growth zone contains two types of secretory vesicles probably involved in the formation of the tube wall. The sub-apical zone, in addition to the secretory vesicles, shows the presence of dilated rough endoplasmic reticulum (RER) cisterns containing electron-dense material which does not react to the test for polysaccharides. In the nuclear zone the vegetative nucleus and the generative cell can be observed; this zone is followed by the vacuolization and callosic plug formation one.