The Km, Vmax, and Ki of the entry reaction were determined for three substrates of the beta-methyl galactoside transport system: D-galactose, D-glycerol-beta-D-galactoside, and beta-methyl-D-galactoside. Although the data for D-galactose and D-glycerol-beta-D-galactoside followed simple Michaelis-Menten kinetics, the results for beta-methyl-D-galactoside deviated from Michaelis-Menten kinetics in that the Ki for beta-methyl-D-galactoside inhibition of both of the other two substrates was 10-fold greater than the Km for beta-methyl-D-galactoside entry. Furthermore, two partial mgl- strains retain 56% of the parental level of the beta-methyl-D-galactoside entry reaction, but only 12% of the parental level of transport of the other two substrates. The exit reaction of beta-methyl-D-galactoside was shown to be first order. It was stimulated sixfold when the cells were provided with an energy source. This stimulation required adenosine 5'-triphosphate or a related compound. The exit reaction was not altered by mutations in any of the three cistrons which inactivate the beta-methyl-D-galactoside entry reaction, was not increased by growth in the presence of inducers of the entry reaction, and was not repressed by growth on glucose. The striking differences between the entry and exit reactions suggest that they either use different carriers or that none of the three cistrons which are currently known to code for components of the beta-methyl galactoside transport system code for its membrane carrier.