Plant diseases caused by Ðlamentous fungi result in the loss of crops worth billions of pounds per year. Agrochemical companies are therefore continually trying to discover and develop new and e †ective antifungal compounds which prevent or inhibit the growth of fungal pathogens in order to improve crop quality and yield.1 The mechanistic basis of the growth of Ðlamentous fungi is currently the focus of much research. A major challenge in the future will be to discover novel antifungal targets within the multifactorial process of fungal growth.Tip growth is the main way by which fungi grow, involving the polarized extension of a hypha by means of localized secretion and cell-wall synthesis at the hyphal apex.2,3 In Edinburgh we are using confocal microscopy with a range of vital Ñuorescent probes to analyse hyphal tip growth and its regulation in living cells.4,5 Three aspects being concentrated on are : (1) Spitzenko rper behaviour ; (2) intracellular pH ; and (3) intracellular Ca2`.Growing hyphal tips contain a multi-component organelle assemblage called the Spitzenko rper which is assumed to contain the secretory vesicles responsible for tip growth. Spitzenko rper behaviour is associated intimately with the dynamic growth pattern and morphogenesis of the hyphal tip.5h11 We have developed a Ñuorescent staining procedure which allows the dynamic behaviour of the Spitzenko rper to be visualised (in growing hyphae) for the Ðrst time.5The Spitzenko rper-selective dye we use is FM4-64. This styryl dye stains components of the endocytotic pathway and also, after membrane recycling, exocytotic vesicles. FM4-64 (and related dyes especially FM1-43) has been extensively used to image and analyse membrane trafficking during endocytosis and exocytosis in animal cells,12 and more recently has been used to study endocytosis in yeast.13,14 Using this dye we have stained the Spitzenko rper in growing hyphae of a wide range of Ðlamentous fungi including Aspergillus niger. v.