Twenty-seven yeasts were screened for starch breakdown; the three with the highest rate were strains of Filobasidium capsuligenum, Lipomyces starkeyi and Schwanniomyces occidentalis. Of these, only the last gave mutants with diminished carbon catabolite repression and, hence, enhanced amylase activity. Unlike those yeasts previously reported to break down starch rapidly, these mutants had the commercially advantageous characteristic of growing only slowly on the products of starch break-down and gave rise to readily-inducible auxotrophs. Like hex1 mutants of Saccharomyces cerevisiae, these mutants of Schwanniomyces occidentalis (i) had diminished hexokinase activity, (ii) retained high levels of glucokinase and (iii) resisted carbon catabolite repression of invertase and α-D-glucokinase. In one mutant, isomaltase was induced in the late exponential phase of growth on starch, and this isomaltase was also resistant to repression.