We have studied single molecules and paracrystals of the stalk domain of the microtubule motor protein, kinesin, using circular dichroism, electron microscopy, and optical diffraction. The stalk is a rod-like particle, about 50 nm in length, with about 70% α-helical content (lower than tropomyosin and myosin). These data confirm the previous studies of M. De Cuevas, T. Tao, and L. S. B. Goldstein (J. Cell Biol. 116, 957-966, 1992). The particles also show a tendency to self-associate into dimers or higher aggregates, up to paracrystals with a periodic substructure. Four types of paracrystals have been observed, two with short periodicities (8 and 13 nm, types I and II) and two with periodicities comparable with the subunit length (53-63 nm, type lII and 38 nm, type IV). Types I and II paracrystals can be interpreted to arise from a polar arrangement of subunits with alternating gaps and overlaps and different staggers between adjacent molecules. Type III and IV paracrystals appear to be formed from sets of antiparallel molecules, forming centrosymmetric patterns. The association properties may be important for functions of the kinesin stalk in microtubule- dependent motility.