Protoplasts obtained from Saccharomyces cerevisiae strain LK2G12 by the action of snail enzymes were free of external invertase and did not ferment sucrose, although they still utilized glucose. If small amounts of glucose or fructose were added together with sucrose, sucrose fermentation began after a lag. Within limits, this lag period became shorter as the glucose concentration was increased. Washing of the protoplasts removed the sucrose-fermenting activity, and the washed protoplasts again fermented sucrose only after a lag. Large amounts of invertase were secreted under the conditions of the apparent adaptation. CO 2 production from sucrose was preceded by synthesis and secretion of invertase and accumulation of hexoses. Fermentation of sucrose by the protoplasts does not appear to be a result of increased permeability to sucrose but rather consists in secretion of invertase and fermentation of the hexoses accumulation extracellularly. In the presence of sucrose and glucose, the log-phase cells synthesized large amounts of invertase, most of which remained attached to the cells. With the removal of increasing amounts of cell wall by snail enzyme, an increasing fraction of the newly synthesized invertase was secreted into the medium.