The process of cell division in the ciliated protozoans involves a complex pattern of meiotic and mitotic cell divisions as well as major nuclear restructuring events that comprise a mode of cell division that involves mechanisms of genetic exchange that characterize the unique pattern of sexual reproduction in this primitive eukaryotic microorganism that is called conjugation. This process is initiated in response to stressful environmental conditions. The initial event involves the pairing of two cells of different mating types (I-IV) followed by meiosis and the exchange of the haploid nuclei produced during meiotic cell division. Subsequently, the nucleus of the zygote in each cell engages in several rounds of mitotic cell division.