The amitochondriate parasitic protist Trichomonas vaginalis can utilize either glucose or maltose as carbon and energy source. The mechanisms of maltose utilization were explored with uptake experiments using radio-isotope labelled maltose in combination with the silicone-oil centrifugation technique and enzymatic assays measuring maltose hydrolysis. The uptake of maltose label became saturated after 2-3 h. The uptake of maltose as a function of the external maltose concentration was linear at low concentrations with no further increase at higher levels, kinetics characteristic of reactions obeying Michaelis-Menten kinetics preceded by a diffusion-limited step. Increased viscosity of the medium resulted in decreased maltose uptake, indicating an extracellular location of the diffusion-limited step. Most of the cellular alpha-glucosidase activity of T. vaginalis was detected on the cell surface, suggesting that maltose is hydrolysed to glucose outside the cell. Glucose interfered more with maltose uptake, and maltose less with glucose uptake, than would be expected if 1 mol of maltose were the equivalent of 2 mol of glucose. This pattern of interaction indicated that the interference occurs before the common metabolic pathway and even before the transport step, supporting the idea of extracellular maltose hydrolysis. We conclude that maltose is hydrolysed to glucose in the boundary layer of the cell, a process akin to membrane digestion in vertebrate enterocytes and on the teguments of helminths. The glucose formed is then transported by the glucose carrier of the organism.