Controlling interactions between building blocks, in either guided- or self-assemblies, is becoming increasingly important for the creation of functional materials. We have focused our attention on the well-known model assembly, the filamentous bacteriophage, where our strategy is to selectively alter surface features by focusing on spatially distinct capsid proteins. Towards introducing stimulus-responsive behavior in these flexible, rod-like particles, we have introduced elastin-like polypeptide (ELP) motifs of isoleucine and tyrosine guest residues by recombinant DNA methods. Our hypothesis is that modification of the major coat capsid protein would be greatly amplified by the 2700 copies per particle. Characterization of ELP-phage particles was carried out by microbiological assays, zeta potential, dynamic light scattering, and calorimetry. Bacteria producing ELP-phage particles grow more slowly and surprisingly, ELP-modified phages display a significant reduction in viral infectivity. For the lengths of ELP inserts studied, modified phages do not aggregate from solution as monitored by DLS. However, the hydrodynamic size of the phages depends on the details of the ELP motif. Zeta potential measurements reveal the particles are electrostatically stabilized, and this contributes in part to the energetic barrier against aggregation. Preliminary calorimetric data indicate subtle thermal transitions in the range 35–45°C, suggesting that the ELP motif may collapse without triggering macroscopic aggregation. The results are consistent with the classical picture of critical solution phenomena at low concentrations, where to drive phase separation, solvent quality must be increasingly poor. Apart from being model systems to study basic questions of self-assembly, extending these modular systems is likely to result in improved understanding and control over self-assembly in various applications.