Abnormal phenotypes resulting from haploinsufficiency (HI) are due to the loss of one allele. Recent studies in budding yeast have shown that HI originates from insufficient protein levels or from a stoichiometric imbalance between subunits of protein complexes. In humans, however, HI often involves transcription factors. Therefore, the species differences in HI and the molecular mechanisms of species-specific HI remain under investigation. In this study, HI in fission yeast was systematically surveyed. HI in fission yeast affected genes related to signaling and to basic cellular processes, as observed in budding yeast. These results suggest that there are species differences in HI and that the HI that occurs in fission yeast is intermediate to and HI in budding yeast and humans.