The yeast retrotransposon, Ty1, produces a macromolecular structure known as a virus-like particle (VLP) as an essential part of its replication cycle. The Ty1 Gag-like structural protein TYA, p1-440, alone is capable of directing assembly of the VLP. In order to determine the TYA sequences required for assembly, we have produced a series of truncated and deleted TYA forms and assessed their ability to assemble into particles. Removal of 100 amino acids from the C-terminus renders the TYA protein, p1-340, incapable of particle assembly; however, p1-363 with 77 residues missing from the C-terminus is capable of assembly. Removal of 40 amino acids from the N-terminus (p41-440 and p41-381) does not affect particle formation but more severely N-truncated forms, p71-381 and p100-381, are present as large aggregates within the cells and are therefore either incapable of or unavailable for VLP formation. Analysis of an internally deleted TYA, p1-381 delta 62-114, has identified this as a possible region of the TYA protein important for subunit:subunit interactions during the particle assembly.