A series of ABA triblock copolymers consisting of a central β-sheet polypeptide block and poly(ethylene glycol) (PEG) end blocks were constructed in order to investigate the effect of PEG chain length on assembly behavior. The polypeptide block, consisting of tandem repeats of -(AG)3EG- (A = alanine, G = glycine, and E = glutamic acid), was prepared by E. coli expression and subsequently conjugated via two flanking cysteine residues to maleimide-functionalized PEGs of various chain length. Infrared spectroscopy showed that no major effect of PEG chain length on polypeptide folding occurred. With atomic force microscopy a fibrillar microstructure was observed for all conjugates, with fibrillar heights of ∼2 nm. Only at the highest molecular weight of PEG (5000 g/mol) could an influence on assembly be noticed by the appearance of shorter fibers when compared to the hybrid block copolymers containing the lower molecular weight PEG chains. It can be concluded that this class of peptide-based materials is well capable of fibril formation in the β-sheet stacking direction, whereas the PEG chains prevent their further side-to-side aggregation without interfering strongly with the desired β-sheet interactions.