We report on the growth of molecular nanoribbons from a pi-conjugated polymer. A polydisperse, soluble, end-functionalized poly(para-phenyleneethynylene) (PPE) derivative can self-assemble from solution on flat solid substrates into nanoribbons with a molecular cross-section and a length on the micrometer scale. By using different solvents it has been possible to produce either predominantly randomly oriented ribbons or to align them along the 3-fold symmetry axes of crystalline mica substrates. These nanostructures may be useful for the fabrication of molecular nanowires. Their growth behaviour suggests that the self-assembly of PPE in thin films from solutions is usually governed kinetically.