The system for uptake of maltose in Pseudomonas fluorescens W was inducible. Using a mutant strain unable to hydrolyze maltose, it was shown that maltose was taken up unaltered against a concentration gradient. Uptake of 14C maltose was only significantly inhibited by nonradioactive maltose or maltotriose. These were the only sugars that could displace accumulated radioactive maltose in the strain unable to hydrolyze maltose. Uptake exhibited saturation kinetics and was inhibited by energy poisons, indicating that this system was one of active transport. Sulfhydryl-binding reagents reversibly inhibited maltose uptake. No transport ability was lost when cells were subjected to osmotic shock. Using the protein-binding dye 7-diazonium-1, 3-naphthalene disulfonate a protein or proteins located in or external to the cell membrane was implicated in maltose transport. The hydrolysis of p-nitrophenyl-alpha-D-glucoside (PNPG) was used as an indirect measure of transport ability since penetration of PNPG, not its hydrolysis, was the rate-limiting step.