The growth of the nucleus and cytoplasm, and the cytoplasm-nucleus ratio (CNR) were studied in the alga Hydrodictyon reticulatum Lagerch. The nucleus divides at the maximum values of CNR; CNR decreases between divisions. The synthesis of the nuclear matter takes place immediately after the mitosis, the growth of the cytoplasm prevails before the nuclear division. This unusual phenomenon can be presumably explained by the fact that Hydrodictyon is a polynucleate organism.