A wild strain of Klebsiella aerogenes (V9A), which contains a plasmid Fklac carrying the genes of the lac operon, gives two mutant cell types, recognized by the appearance of their colonies on MacConkey lactose agar. These are referred to as ML− and ML−/+, wild-type being ML+. ML− cells can grow rapidly on 1% lactose as carbon source but very slowly on 0·2% unless induced by TMG or D-fucose, or by previous growth on galactose, melibiose or raffinose, which enables them to grow rapidly on 0·2% lactose for 2–4 cell generations. Previous growth on 1% lactose does not induce the ability to grow rapidly on 0·2% lactose. It is concluded that ML− cells have a defect in the lactose permease gene which allows slow uptake of lactose when the external concentration is 0·2% and more rapid uptake when the external concentration is increased. In addition, TMG, D-fucose, galactose, melibiose and raffinose induce one or more other galactoside permeases which can accumulate lactose efficiently but are not induced by lactose. ML−/+ cells can grow normally on 0·2% lactose as carbon source, but only after a substantial lag when transferred from glucose, glycerol or sucrose, and after an even longer lag when transferred from melibiose or raffinose. Wild-type cells (ML+) grow normally on 0·2% lactose after a short lag of less than a cell generation time, when transferred from any other carbon source. Cells of each of the three phenotypes (+, −/+ and −) can mutate to give both of the other two phenotypes. Incomplete genetic evidence suggests that the ML− mutation is a result of a reversible change in the Fklac plasmid.