The fission yeast Schizosaccharomyces pombe has a haplontic life cycle in which the diplophase is confined to the zygote. Through the use of one- and two-step protoplast fusions we show that the ploidy can be increased up to pentaploid. The polyploid fusion products are rather unstable and segregate cells of lower ploidies by gradual loss of chromosomes during mitotic divisions. The polyploid cells conjugate normally but are prone to arrest at various stages of meiosis (1-, 2- and 3-spored asci, binucleate spores) and/or produce inviable, most probably aneuploid, spores. Marker segregation in the complete tetrads indicates the multiple association of homologous chromosomes. In tetra- and penta-ploid meiosis, multispored (6- to 7-spored) asci are also produced, probably by postmeiotic division of the nuclei.