Motion-whether it the ability to change shape, rotate or translate-is an important potential asset for functional nanostructures. For translational motion, a variety of DNA-based and small-molecule walkers have been created, but observing the translational motion of individual molecules in real time remains a significant challenge. Here, we show that the movement of a small-molecule walker along a five-foothold track can be monitored continuously within a protein nanoreactor. The walker is an organoarsenic(III) molecule with exchangeable thiol ligands, and the track a line of cysteine residues 6 angstrom apart within an alpha-haemolysin protein pore that acts as the nanoreactor. Changes in the flow of ionic current through the pore reflect the individual steps of a single walker, which require the making and breaking of As-S bonds, and occur in aqueous solution at neutral pH and room temperature. The walker moves considerably faster (similar to 0.7 s per step) than previous walkers based on covalent chemistry and is weakly processive (6 +/- 1 steps per outing). It shows weak net directional movement, which can be described by a thermodynamic sink arising from the different environments of the cysteines that constitute the track.