Alzheimer’s disease is the most common neurodegenerative disease, affecting ∼50% of humans by age 85. The disease process is associated with aggregation of the Aβ peptides, short 39–43 residue peptides generated through endoproteolytic cleavage of the Alzheimer’s precursor protein. While the process of aggregation of purified Aβ peptides in vitro is beginning to be well understood, little is known about this process in vivo. In the present study, we use the yeast <i>Saccharomyces cerevisiae</i> as a model system for studying Aβ-mediated aggregation in an organism in vivo. One ofthis yeast’s endogenous prions, Sup35/[<i>PSI</i>⁺], loses the ability to aggregate when the prion-forming domain of this protein is deleted. We show that insertion of Aβ peptide sequences in place of the original prion domain of this protein restores its ability to aggregate. However, the aggregates are qualitatively different from [<i>PSI</i>⁺] prions in their sensitivity to detergents and in their requirements on <i>trans</i>-acting factors that are normally needed for [<i>PSI</i>⁺] propagation. We conclude that we have established a useful new tool for studying the aggregation of Aβ peptides in an organism in vivo.