Application of Fourier analysis techniques to images of isolated, frozen-hydrated subpellicular microtubules from the protozoan parasite Toxoplasma gondii demonstrates a distinctive 32 nm periodicity along the length of the microtubules. A 32 nm longitudinal repeat is also observed in the double rows of intramembranous particles seen in freeze-fracture images of the parasite's pellicle; these rows are thought to overlie the subpellicular microtubules. Remarkably, the 32 nm intramembranous particle periodicity is carried over laterally to the single rows of particles that lie between the microtubule-associated double rows. This creates a two-dimensional particle lattice, with the second dimension at an angle of approximately 75 degrees to the longitudinal rows (depending on position along the length of the parasite). Drugs that disrupt known cytoskeletal components fail to destroy the integrity of the particle lattice. This intramembranous particle organization suggests the existence of multiple cytoskeletal filaments of unknown identity. Filaments associated with the particle lattice provide a possible mechanism for motility and shape change in Toxoplasma: distortion of the lattice may mediate the twirling motility seen upon host-cell lysis, and morphological changes observed during invasion.