A strain of fission yeast carrying replicating instability (RI) will segregate mitotically three types of cells: unstable (still RI-carrying) cells, stable identical mutants and stable non-mutants. RI in fission yeast has previously been considered as a specific type of premutational lesion capable of (1) being replicated as such and (2) reverting at an appreciable rate to the normal state as well as changing into a stable mutation. In the present work genetic analysis of a previously studied RI-carrying strain showed this strain to be a diploid, most probably heterozygous for a recessive mutation. It was possible to construct other unstable heterozygous strains segregating predetermined mutants but not to induce RI by UV-irradiation of haploid cells. Thus evidence is presented against the premutational nature of RI in fission yeast.