The construction of diploid cells eases genetic analysis in haploid genetic systems because diploid cells allow for the characterization of essential genes. Here, we report the construction of diploid cells using ade6 point mutants that suppress each other via interallelic complementation in the fission yeast Schizosaccharomyces japonicus var japonicus (Sz. japonicus). We constructed an ade6-domK mutant in addition to the previously described ade6-domE. Phenotypes of both mutants exhibited adenine auxotrophy and red colonies. The mutations complemented the phenotypes in a mutually dependent manner. Diploid zygotes, in which the two mutations were introduced simultaneously into the same cells, were isolated by selecting for adenine independence. Such diploid cells are apparently larger in size than haploid cells, yet have a similar nuclear/cytoplasmic ratio, and thus the nuclear size control that has been reported in Sz. pombe is also present in Sz. japonicus. Copyright (C) 2011 John Wiley & Sons, Ltd.