In this study, the filamentous green alga Zygogonium ericetorum (Zygnematales, Chlorophyta), collected at its natural habitat in the high alps, was investigated by light, scanning, and transmission electron microscopy. The field samples were separated into a moist fraction when wetted by splattering water of a nearby spring or a desiccated one when visually dried out. Light microscopy demonstrated a purple pigmentation of the sun-exposed upper layers, the central position of the nucleus, and the starch content in the pyrenoids. The smooth surface of the cells occasionally covered with fungal hyphae was shown by scanning electron microscopy. The cytoarchitecture of moist cells revealed many vacuoles and only a thin cytoplasmic area surrounding the two chloroplasts. The secondary cell walls of older cells were up to 4 microm thick. Organelle membranes as well as thylakoid membranes occasionally showed an inversion of contrast. In the chloroplasts, distinct areas with granular content surrounding the pyrenoids were detected. Within the cytoplasm, electron-dense particles with electron-translucent crystalloid structures were observed. When desiccated samples were investigated, the vacuoles and cytoplasmatic portions appeared destroyed, whereas nucleus and chloroplasts generally remained intact. The thylakoid membranes of desiccated samples showed lumen dilatations and numerous plastoglobules. Water-soluble extracts were separated by high-pressure liquid chromatography that revealed two major compounds with UV-absorbing capacities.