Two mutants of Streptococcus lactis ATCC 11454 have been isolated which possess an impaired lactose-fermenting capacity; galactose utilization is also affected, but to a lesser extent. Although the Embden-Meyerhof-Parnas pathway is the major, if not the sole, pathway of carbohydrate metabolism in the three strains, the fermentation end products of the mutants are dramatically different from the typical homolactic pattern of the wild type. Under conditions of low oxygen tension and growth-limiting lactose concentrations, mutant strain T-1 produces largely formic acid, acetic acid (2:1), and ethanol rather than lactic acid. Aerated cultures produce acetic acid, CO(2) (1:1), acetyl-methylcarbinol, and diacetyl. When the mutants use galactose as an energy source, lactic acid is the major end product, but significant heterofermentative activity is observed. The aberrations responsible for the mutant phenotypes reside in the proteins which catalyze the transport and hydrolysis of galactosides. It is hypothesized that the impaired transport system of the mutants reduces the intracellular pool of glycolytic intermediates below that of the wild type. Since fructose-1, 6-diphosphate is an activator of lactic dehydrogenase in S. lactis, lactic acid production is reduced, and pathways leading to the formation of other products are expressed.