The membrane-selective fluorescent dye FM4-64, N-(3-triethylammoniumpropyl)-4-(6-(4-(diethylamino)phenyl)hexatrienyl)py ridium dibromide, was used to stain the apical vesicle cluster within the specialized Spitzenkorper of the germ tube of the rust fungi Uromyces vignae and Puccinia graminis f. sp. tritici grown on glass surfaces. The Spitzenkorper stained within 15 min following addition of the dye. Optical sectioning by confocal microscopy of stained hyphal tips showed that the Spitzenkorper was asymmetrically positioned close to the cell-substratum interface during germ tube growth. The Spitzenkorper showed variations in shape and positioning over short (5 s) time intervals. The movement to a new location in the hyphal dome was followed by new growth in that region, consistent with the view that the Spitzenkorper supplies secretory vesicles for germ tube growth. A pronounced Spitzenkorper disappeared at the onset of appressorium differentiation during swelling of the germ tube. However, a stained structure, similar in appearance to a Spitzenkorper, was again observed during the formation of the highly polarized penetration peg.