All stages of vertical transmission of symbiotic bacteria, from the penetration into oocytes to the for-mation of rhagon, were investigated in the White Sea (Arctic) representatives of Halisarca dujardini Johnston (Demospongiae). Small populations of free-living spe-cific symbiotic bacteria inhabit the mesohyl of H. du-jardini. They are represented by a single morphotype of small spiral gram-positive bacteria. Vertical transmis-sion of symbiotic bacteria between generations in sponges may occur in different ways. In the case of H. dujardini the bacteria penetrate into growing oocytes by endocytosis. A part of the bacteria plays a trophic role for oocytes and the other part remains undigested in membrane-bound vacuoles within the cytoplasm. In cleaving embryos bacteria are situated between the blastomeres or in the vacuoles. In the blastula all bac-teria are disposed in the blastocoel. The symbionts are situated in intercellular spaces in free-swimming larvae and during metamorphosis. Symbiotic bacteria do not play any trophic role in the period of embryonic and postembryonic development of H. dujardini. No signs of destruction and digestion of bacteria were revealed at any stage of development.