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The first enzyme in the biosynthesis of leucine in yeast, a-isopropylmalate synthetase, is inhibited by L-leucine. In a mutant resistant to the analogue 5',5',5'trifluoroleucine, the enzyme is markedly resistant to inhibition by L-leucine. Growth ing the presence of exogenous L-leucine results in repression of the second and third enzymes of the pathway. The first enzyme is not repressed unless both L-leucine and L-threonine are supplied in the medium. Comparison of levels of the remaining two enzymes in leucine auxotrophs grown under conditions of leucine excess and leucine limitation reveals deviations from the wild-type derepression pattern in some mutants. In some, repression of the synthetase by leucine alone was observed. In others, the repressibility of the dehydrogenase was lost. It is unlikely that these deviations were due to the same primary mutational event that caused leucine auxotrophy. No mutants were found in which an altered gene was recognized to be clearly responsible for the level of the leucine-forming enzymes.