By use of a selective galactose agar medium containing ethidium bromide, a population analysis of the deinduction kinetics of yeast galactose long-term adaptation mutants (gal 3) has been done. It was first determined that the gal 3 mutation is specific to the yeast galactose system and that induced cultures of gal 3 strains are capable of growth on galactose agar medium containing ethidium bromide, whereas noninduced cultures are not. Population analyses of induced gal 3 strains under going deinduction in the absence of galactose demonstrate that a minimum number of five induction units per cell are required for induction of the galactose system. It is concluded that: these induction units are actively synthesized only in the presence of inducer and are diluted out through cell division; they are stable under nongrowing conditions; they are heterogeneous in nature; at most two of the five minimum units are products of the gal 2 locus; and the other units may be three of one type, one of one type and two of another, or one each of three different types.