Low-dose-high-resolution electron microscopy (LD-HREM) was used to analyze the structure of regularly twisted poly(nz-phenylene isophthalamide) (MPDI) strands formed by slow crystallization from solution. The MPDI chains were found to aggregate into regular assemblies exhibiting uniform twisting at several different length scales ranging from the molecular to the mesoscopic. The MPDI polymer backbone formed a flattened helical structure that was organized into twisted bundles. The helical molecular structure promoted good lateral packing, but led to an open core running down the helical axis, which is presumably filled with solvent and calcium chloride. The electron diffraction and HREM data were consistent with a pseudohexagonal unit cell with a = b = 1.65 +/- 0.02 nm, gamma = 120 +/- 0.5 degrees, and c = 0.378 nm, where the c-axis was oriented nominally parallel to the twist axis of the fiber. The strain induced by the mesoscopic twisting is compensated for by lateral shift disorder between the polymer helices, as confirmed by direct HREM imaging.