The microtubule-associated tau proteins become functionally and structurally altered in Alzheimer's disease (AD). To analyze tau modification and its role in a non-vertebrate animal model, we produced transgenic Caenorhabditis elegans strains with a panneuronal expression of human tau and a pseudohyperphosphorylated (PHP) tau construct that mimics AD-relevant tau modification. We show that human tau in C. elegans becomes highly phosphorylated and exhibits conformational changes similar to PHP tau and human PHF tau. Both, wt tau and PHP tau induced a progressive age-dependent development of a phenotype of uncoordinated locomotion (unc) in the absence of neuronal degeneration. However, only PHP tau induced a defective pattern of motor neuron development as indicated by the presence of gaps in the dorsal cord, commissures on the wrong side and local broadening of axons. The data indicate that C. elegans is capable of highly phosphorylating human tau to an AD-like state whereas only stable disease-like tau modification induce developmental defects suggesting a specific interference of pathologic tau with intracellular mechanisms of axonal outgrowth and pathfinding.