SUMMARY: The formation of invertase in Saccharomyces fragilis was studied by the continuous culture technique. Invertase formation was markedly inhibited by glucose in the culture medium at concentrations greater than 0·001% (w/v), by galactose and lactose at all concentrations tested, and by several other carbon sources. The mean generation time of the organisms and the concentration of ammonia and growth factors had no significant effect on invertase formation. The pH value of the growth medium modified the amount of enzyme formed. The invertase activity, together with non-enzymic hydrolysis of sucrose by the acidic culture medium, was sufficient to account for the disappearance of the observed amount of sucrose from media containing sucrose.