SUMMARY

The flagellum of Euglena gracilis has been studied comparatively, using the freeze-fracture technique and with thin sections involving a tannic acid staining procedure. The flagellar membrane exhibits specializations in the form of two groups of intramembrane particle rows (IMP rows; mean IMP diameter, 6 nm) and single clusters of larger IMPs (mean IMP diameter, 10 nm) associated with the edge of one of the two groups of IMP rows. The IMP rows are helically arranged. The two groups of IMP rows are separated from each other by two membrane areas (measuring 170 and 300 nm in length) devoid of IMP arrays. The IMP rows are arranged in a four-start helix around the flagellum. A comparative morphometric analysis of IMP arrays and the arrangement of the two types of non-tubular mastigonemes (evaluated from tannic-acid-stained thin sections) reveals that the two groups of IMP rows correspond to the attachment points of the short non-tubular mastigonemes, whereas the cluster of three to seven larger IMPs corresponds to the attachment points of a group of the long non-tubular mastigonemes. Flagellar membrane specializations also occur with respect to sterols, since filipin-sterol complexes are absent from all plasma membrane regions except the flagellar membrane. O