Pneumocystis species are obligate fungal biotrophs that colonize the lungs of mammals. They cause deadly pneumonia in immunocompromised hosts. The sexual phase seems obligate during their life cycle and essential for survival because it is believed to ensure proliferation and transmission between hosts. Here, we consider if the sexual phase is initiated by the fusion of two cells or by nucleus duplication in order to generate diploid cells that can undergo meiosis. The juxtaposition of the nucleus-associated organelles of pairs of cells with fused cytoplasmic membranes demonstrated that cell fusion can occur. Nevertheless, the frequency of cell fusion remains to be determined, and it cannot be excluded that both cell fusion and nucleus duplication are used to ensure the occurrence of the essential sexual phase. In vitro culturing of these fungi is a major milestone that could clarify the issue.