In a previous report we have shown that microtubule-associated protein tau can be induced to form paracrystals (Lichtenberg, B., E.-M. Mandelkow, T. Hagestedt, and E. Mandelkow. 1988. Nature [Lond.]. 334:359-362). A striking feature was the high degree of elasticity of the molecules. We now report that this property is related to the state of phosphorylation. When tau is dephosphorylated by alkaline phosphatase, it becomes shorter and more elastic; when it is phosphorylated by Ca++/calmodulin-dependent kinase, it becomes longer and stiffer. This may provide a model for the control of structural properties of tau-like molecules by phosphorylation.