Cis-acting regulatory mutations have been isolated that affect L-ornithine transaminase (OTAse), an enzyme catalyzing the second step of arginine breakdown in yeast. These mutations lead to constitutive synthesis of OTAse at various levels. Two different types of mutations have been recovered, both of which are tightly linked to the structural gene (cargB) for this enzyme. One type behaves as a classical operator-constitutive mutation similar to the cargB+O---1 mutation previously described (DUBOIS et al. 1978). The second type is peculiar in two respects: the higher level of constitutive OTAse synthesis and the expression of constitutivity in diploid cells. These mutations are designated cargB+Oh. They behave as usual operator-constitutive mutations in diploid strains homozygous for mating type (a/a or alpha/alpha), but the constitutivity is strongly reduced in a/alpha diploid cells.