During routine screenings of random peptide libraries displayed at the N terminus of the pIII coat protein of M13 bacteriophage, clones were isolated that bound directly to the polystyrene (PS) surface used to immobilize the target protein. The plastic-binding phage (P-b phi) bind to both unblocked plastic (PS and polyvinyl chloride, PVC) and plastic blocked with bovine serum albumin (BSA) but require non-ionic detergent to bind to plastic blocked with milk. Comparison of the P-b phi to antibody-binding phage (Ab-b phi) indicates that similar numbers of phage particles are bound, but fewer P-b phi the recovered by acid elution. Sequence determination of the displayed peptides reveals they lack amino-acid sequence similarity yet are highly enriched for the Tyr and Trp residues. However, because not all phage that display peptides rich in Tyr and Trp residues bind to plastic, and other methods of screening random peptide libraries have identified different classes of plastic-binding peptides, the relative abundance of Tyr and Trp residues should not be considered diagnostic of plastic-binding. In summary, these results help characterize one of the most common methods used to screen random peptide libraries and suggest strategies to avoid isolating P-b phi. Furthermore, while it is generally believed that proteins bind to plastic by non-specific interactions, these results show that a bias in aa composition can exist.