Electron microscopical studies of the nucleoid structure of hydrogen bacteria using ultrahin sections and spread DNA from bacterial cell lysates revealed a different DNA packaging in the cell. A compact state of the major part of DNA at all growth stages and stability of nucleosome-like structures were shown. The use of antibodies to HU protein of E. coli labelled by protein A-colloidal gold demonstrated the immunological relationship between HU protein of E. coli and histone-like proteins of Alcaligenes eutrophus and their possible role in the nucleosome-like DNA packaging in procariotic genome.