Abstract A theoretical description of initial uptake kinetics of H +/sugar symport is given, with emphasis on the differences between carrier and non-carrier systems. Transport of methyl β- d-thiogalactoside in Saccharomyces fragilis is shown to proceed via the inducible lactose transporter. Uptake of this sugar stimulates electrogenic H + influx. Together with the correlation between methyl β- d-thiogalactoside accumulation and the proton-motive force this shows that transport proceeds via H + symport. Kinetic analysis of initial influx revealed that transport proceeds via a single transport system, sensitive to changes in membrane potential. The pH dependence of the kinetic parameters showed that K app is almost pH insensitive, whereas V app decreases strongly at increasing extracellular pH. It is shown that transport proceeds, most likely, via a non-carrier system, with random binding of H + and sugar, in a system where binding of the first ligand does not influence binding of the second.