The Papilio xuthus (Lepidoptera: Papilionidae) pupa expresses novel soluble proteins that undergo reversible temperature-dependent coacervate-formation. We purified two coacervate-forming proteins, PX-1 and PX-4, from the wings of pharate adults. PX-1 and PX-4 form coacervates upon warming. Transmission electron microscopy analysis revealed that these proteins assemble ordered bead-like ultrastructures. We cloned and sequenced PX-1 and PX-4 cDNAs. The PX-1 and PX-4 amino acid sequences contain many hydrophobic residues and show homologies to insect cuticular proteins. Moreover, when recombinant PX-1 and PX-4 were overexpressed in Escherichia coli, both recombinant proteins exhibited temperature-dependent coacervation. Furthermore, analyses of truncated mutants of PX-1 suggest that both the Val/Pro-rich region and Gly/lle-rich regions of PX-1 are involved in such coacervation.