SUMMARY: The flagella of certain large spirilla appear to be compound structures composed of a large number of individual fibres. These arise in bundles from a single blepharoplast , whereas in other bacteria each flagellum arises separately. The flagella of spirilla are thus, in some respects, intermediate between those of other bacteria and of flagellates. Since the description of a ' thread-like trunk ' on cells of Ophidomonas (Thio- spirillum) spp. which behaved as an organ of propulsion (Ehrenberg, 1838) numerous observations have been made on the flagella of both living and stained cells of large spirilla (Biitschli, 1902 ; Ellis, 1902; Fuhrmann, 1909; Reichert, 1909; Buder, 1915; Yuasa, 1936; Pijper, 1949). These have con- firmed that most spirilla possess a large number of very fine flagellar strands, but have not adequately differentiated these complex structures from the multiple, lophotrichous flagella of, for example, Pseudornonas spp. Several workers (Butschli, 1902; Fuhrmann, 1909 ; Yuasa, 1936) suggested that the flagella arose from a single basal structure analogous to the blepharoplast of the flagellates. More recent studies with the electron microscope (van Iterson, 1947, 1953; Lofgren, 1948; Houwink, 1953) have shown that the origin of the flagella is intracellular but have not elucidated completely their mode of attachment. Van Iterson (1953) failed to find any evidence of a blepharoplast after autolysis of the cells, whereas Lofgren (1948) described a disk-like structure to which the flagellar strands were attached. Houwink (1953) observed a small hook at the base of each flagellar strand similar to those on the flagella of other bacteria. Blepharoplasts have, however, been observed in other bacterial groups, notably Proteus (Houwink & van Iterson, 1950), Vibrio (van Iterson, 1953), Pseudornonas and Bacterium spp. (Bisset & Hale, 1951). In these it was shown that each flagellum arose from a separate granule. From their structure, as seen by the electron microscope, and from their behaviour, the flagella of large spirilla appear to be compound structures consisting of numerous slender fibrils which arise from a single blepharoplast and function as a single organ in a manner analogous to that of the simple, but much stouter flagellum of, for example, Vibrio spp. They are, thus, distinct from the multiple, separate flagella of other bacteria. METHODS