A human intestinal spirochete isolated from a rectal biopsy specimen was morphologically characterized. The isolate was comma-shaped, 3-6 microm in length, 0.2 micro m in diameter and had tapered ends. The surface layer, external to the outer envelope, was amorphous. Four string-like periplasmic flagella with a diameter of 20 nm were presented at each end of the SDS-treated cells. Thin sections of the bacterial cell revealed a high-density cytoplasmic membrane and flagella in the periplasmic space between the cytoplasmic membrane and the outer envelope. Three segments of equal length were observed in some of the cells, while other cells were bi-segmented and more frequently observed.