The [URE3] non-Mendelian genetic element of Saccharomyces cerevisiae is an infectious protein (prion) form of Ure2p, a regulator of nitrogen catabolism. Here, synthetic Ure2p(1-65) were shown to polymerize to form filaments 40 to 45 angstroms in diameter with more than 60 percent beta sheet. Ure2p(1-65) specifically induced full-length native Ure2p to copolymerize under conditions where native Ure2p alone did not polymerize. Like Ure2p in extracts of [URE3] strains, these 180- to 220-angstrom-diameter filaments were protease resistant, The Ure2p(1-65)-Ure2p cofilaments could seed polymerization of native Ure2p to form thicker, Less regular filaments. All filaments stained with Congo Red to produce the green birefringence typical of amyloid. This self-propagating amyloid formation can explain the properties of [URE3].