Summary The ability of a yeast cell to propagate PSI + , the prion form of the Sup35 protein, is dependent on the molecular chaperone Hsp104. Inhibition of Hsp104 function in yeast cells leads to a failure to generate new propagons, the molecular entities necessary for PSI + propagation in dividing cells and they get diluted out as cells multiply. Over‐expression of Hsp104 also leads to PSI + prion loss and this has been assumed to arise from the complete disaggregation of the Sup35 prion polymers. However, in conditions of Hsp104 over‐expression in PSI + cells we find no release of monomers from Sup35 polymers, no monomerization of aggregated Sup35 which is not accounted for by the proportion of prion‐free psi ‐ cells present, no change in the molecular weight of Sup35‐containing SDS‐resistant polymers and no significant decrease in average propagon numbers in the population as a whole. Furthermore, they show that over‐expression of Hsp104 does not interfere with the incorporation of newly synthesised Sup35 into polymers, nor with the multiplication of propagons following their depletion in numbers while growing in the presence of guanidine hydrochloride. Rather, they present evidence that over‐expression of Hsp104 causes malpartition of PSI + propagons between mother and daughter cells in a sub‐population of cells during cell division thereby generating prion‐free psi − cells.