Using Feulgen's method and light microscopy, an extensive cytogenetical study concerning nuclear and chromosomal structure, mitosis and meiosis in mesokaryotic algae was performed. A tentative karyotype in a mesokaryotic alga has been done for the first time. A particular mechanism of sexuality in which an unidirectional transfer of genetic material takes place is described inPeridinium trochoideum.The behaviour of eukaryotic nucleus in the division process ofP. balticum, P. tabulatumandP. trochoideumis also described. It is concluded that eukaryotic nucleus may represent a parasite of genetic level.