SUMMARY: The study of ultrathin sections of Thiobacillus novellus has shown that it has a structure similar to that of T. thiooxidans and other Gram-negative bacteria. The cell wall showed a multilayered structure and varied in thickness between 250 and 350 Å. The cytoplasmic membrane and 70 Å thick. On certain media a capsule was formed. The cytoplasm contained ribosomes and polymetaphosphate granules. The structure of the nuclear material was similar to that observed in other bacteria.