In this work, we have studied the effect of amplifying different alleles involved in the threonine biosynthesis on the amino acid production by Saccharomyces cerevisiae. The genes used were wild-type HOM3, HOM2, HOMS, THR7, and THR4, and two mutant alleles of HOM3 (namely HOM3-RZand HOM3-R6), that code forfeedbackinsensitive aspartate kinases. The results show that only the amplification of the HOM3 alleles leads to threonine and, in some instances, to homoserine overproduction. In terms ofthe regulation ofthe pathway, the data indicate that the main control is exerted by inhibition of the aspartate kinase and that, probably, a second and less important regulation takes place at the level of the homoserine kinase, the THR7 gene product. However, amplification of THR7 in two related Hom3-R2 strains does not increase the amount of threonine but, in one of them, it does induce accumulation of more homoserine. This result probably reflects differences between these strains in some undetermined genetic factor/s related with threonine metabolism. In general, the data indicate that the common laboratory yeast strains are genetically rather heterogeneous and, thus, extrapolation of conclusions must be done carefully.