Biological pumps that use retinal isomerization to move protons across a membrane have been studied extensively, but the mechanisms involved in moving chloride ions, which have both a different charge and different coordination requirements, are less well understood. Mous et al. combined time-resolved x-ray crystallography, spectroscopy, and computational simulations to generate a molecular movie of chloride transport through a chloride-pumping halorhodopsin. Ion uptake from the extracellular environment is supported by interactions with the retinal, and transport through a space generated by retinal isomerization occurs within about 1 microsecond of excitation. Release of chloride and blockage of backward flow is mediated by a salt bridge that forms an electrostatic gate at the intracellular face. These insights are important for understanding ion transport in these pumps, which are important tools for optogenetic silencing of neurons. —MAF