Carbon dioxide stimulates the growth of Streptococcus bovis and the production of the extracellular enzyme dextransucrase. The growth of cultures in the presence of C14O2 resulted in the incorporation of carbon-14 into aspartic acid, with lesser amounts in threonine, glutamic acid, adenine, guanine, uridylic acid and cytidylic acid. Organisms grown with labelled aspartic acid did not utilize it to any extent; the principal source of this amino acid was from CO2-fixation reactions. The stimulation of growth by CO2 is considered to be due to biosynthetic reactions involving CO2 which lead to a sufficient supply of the intermediates required in nucleic acid and protein synthesis.