1. It was confirmed that the colourless alga, Prototheca zopfii converts glucose completely to lactic acid during fermentation. 2. Lactic acid was the only acidic product of glucose fermentation by Chlorella strain 211/1e, 1.4 moles being formed for each mole glucose fermented. This strain produced negligible carbon dioxide and hydrogen; other fermentation products were not identified. 3. In regard to capacity for fermenting glucose, the Chlorella strains examined fell into 4 groups, namely (a) those producing about 0.75 moles lactic acid/mole glucose (strains 8b, 8p, 11f, 11n), (b) those producing about 1.3 mole lactic acid/mole glucose (strains 1e and 11b), (c) those producing acid but not lactic acid (strain 11g), (d) those unable to ferment glucose (strains 1a, 1d and 9a). This grouping of strains agrees well with groupings based on other biochemical criteria.