Chain-length polydispersity is among the least understood factors governing the fibrillation propensity of homopolypeptides. For monodisperse poly-L-glutamic acid (PLGA), the tendency to form fibrils depends of the main-chain length. Long-chained PLGA, so-called (Glu)(200), fibrillates more readily than short (Glu)(5) fragments. Here we show that conversion of alpha-helical (Glu)(200) into amyloid-like beta-fibrils is dramatically accelerated in the presence of intrinsically disordered (Glu)(5). While separately self-assembled fibrils of (Glu)(200) and (Glu)(5) reveal distinct morphological and infrared characteristics, accelerated fibrillation in mixed (Glu)(200) and (Glu)(5) leads to aggregates similar to neat (Glu)200 fibrils, even in excess of (Glu)(5). According to molecular dynamics simulations and circular dichroism measurements, local events of "misfolding transfer" from (Glu)(5) to (Glu)(200) may play a key role in the initial stages of conformational dynamics underlying the observed phenomenon. Our results highlight chain-length polydispersity as a potent, although so-far unrecognized factor profoundly affecting the fibrillation propensity of homopolypeptides.