© 2014 American Chemical Society. A group of microbial retinal proteins most closely related to the proton pump xanthorhodopsin has a novel sequence motif and a novel function. Instead of, or in addition to, proton transport, they perform light-driven sodium ion transport, as reported for one representative of this group (KR2) from Krokinobacter. In this paper, we examine a similar protein, GLR from Gillisia limnaea, expressed in Escherichia coli, which shares some properties with KR2 but transports only Na+. The absorption spectrum of GLR is insensitive to Na+at concentrations of ≤3 M. However, very low concentrations of Na+cause profound differences in the decay and rise time of photocycle intermediates, consistent with a switch from a "Na+-independent" to a "Na+-dependent" photocycle (or photocycle branch) at ∼60 μM Na+. The rates of photocycle steps in the latter, but not the former, are linearly dependent on Na+concentration. This suggests that a high-affinity Na+binding site is created transiently after photoexcitation, and entry of Na+from the bulk to this site redirects the course of events in the remainder of the cycle. A greater concentration of Na+is needed for switching the reaction path at lower pH. The data suggest therefore competition between H+and Na+to determine the two alternative pathways. The idea that a Na+binding site can be created at the Schiff base counterion is supported by the finding that upon perturbation of this region in the D251E mutant, Na+binds without photoexcitation. Binding of Na+to the mutant shifts the chromophore maximum to the red like that of H+, which occurs in the photocycle of the wild type.