Two types of large spirochetes, differing in size, and in coiling and motility patterns, were observed in sulfide-containing mud collected from a brackish marsh. The spirochetes measured 50–250 μm in length and 0.5–0.75 μm in diameter. One type was identified as Spirochaeta plicatilis Ehrenberg, inasmuch as its general characteristics corresponded to those described for this species by other authors. The second type of large spirochete was designated “large spirochete type B”. Both types possessed ultrastructural features typical of spirochetes, e.g. outer sheath, axial fibrils and protoplasmic cylinder. The axial fibrils were arranged in a bundle and, in type B spirochetes, were inserted in a row near each end of the cell. The outermost layer of type B cells, interpreted to be the outer sheath or a main component of it, appeared to consist of an array of fibril-like elements which transversely circumscribed the cell. Transverse septa were observed in S. plicatilis, but not in type B spirochetes. In the mud samples both types of spirochetes were present near the mud-water interface within mats of sulfur-containing Beggiatoa trichomes, together with a variety of other microorganisms. A striking increase in the number of large spirochetes consistently occurred during lysis of the Beggiatoa trichomes, at low levels of H2S. The large spirochetes were attracted to the lysing trichomes and grouped about them. Under strictly anaerobic conditions S. plicatilis remained motile for 8 days, and type B for 25 days. Experiments involving the use of chemical and biological indicators showed that S. plicatilis is either an anaerobe which can tolerate low O2 tensions, or a microaerophile able to metabolize and possibly grow in the absence of O2. Type B spirochetes behaved as strict anaerobes.