The self-perpetuating conformational change of the translation termination factor Sup35 is associated with a prion phenomenon of Saccharomyces cerevisiae. In vitro, the prion-determining region (NM) of Sup35 assembles into amyloid-like fibres through a mechanism of nucleated conformational conversion. Here, we describe an alternative assembly pathway of NM that produces filaments that are composed of beta-strands and random coiled regions with several-fold smaller diameters than the amyloid fibres. NM filaments are not detectable with either thioflavin T or Congo Red and do not show SDS or protease resistance. As filaments do not self-convert into fibres and do not act as seed, they are not intermediates of amyloid fibre formation. Instead, they represent a stable off-pathway form. Similar to mammalian prion proteins, Sup35 contains oligopeptide repeats located in the NM region. We found that the number of repeats determines the partitioning of the protein between filaments and amyloid-like fibres. Low numbers of repeats favour the formation of the filamentous structure, whereas high numbers of repeats favour the formation of amyloid-like fibres.