Helical Bacillus subtilis macrofibers are highly ordered structures consisting of individual cells packed in a geometry remarkably similar to that found in helically twisted yarns (G. A. Carnaby, in J. W. S. Hearle et al., ed., The Mechanics of Flexible Fibre Assemblies, p. 99-112, 1980; N. H. Mendelson, Proc. Natl. Acad. Sci. U.S.A. 75:2478-2482, 1978). The growth and formation of macrofibers were studied with time-lapse microscopy methods. The basic growth mode consisted of fiber elongation, folding, and the helical wrapping together of the folded portion into a tight helical fiber. This sequence was reiterated at both ends of the structure, resulting in terminal loops. Macrofiber growth was accompanied by the helical turning of the structure along its long axis. Right-handed structures turned clockwise and left-handed ones turned counterclockwise when viewed along the length of a fiber looking toward a loop end. Helical turning forced the individual cellular filaments into a close-packing arrangement during growth. Tension was evident within the structures and they writhed as they elongated. Tension was relieved by folding, which occurred when writhing became so violent that the structure touched itself, forming a loop. When the multistranded structure produced by repeated folding cycles became too rigid for additional folding, the morphogenesis of a ball-like structure began. The dynamics of helical macrofiber formation was interpreted in terms of stress-strain deformations. In view of the similarities between macrofiber structures and those found in multifilament yarns and cables, the physics of helical macrofiber structure and also growth may be suitable for analysis developed in these fields concerning the mechanics of flexible fiber assemblies (C. P. Buckley; J. W. S. Hearle; and J. J. Thwaites, in J. W. S. Hearle et al., ed., The Mechanics of Flexible Fibre Assemblies, p. 1-97, 1980).