Many locomotor traits exhibit predictable variation along environmental gradients by influencing the lower level cellular and physiological processes that determine whole organism performance. For example, thermal performance curves are commonly used to describe how locomotive performance changes with increasing temperature. Here, we examine how increasing salinity alters internal osmolality and escape performance in two closely related euryhaline fish. On the island of Trinidad, the Trinidadian guppy (Poecilia reticulata) is confined to freshwater whereas the swamp guppy (Poecilia picta), co-exists with P. reticulata in freshwater, but also spans into brackish and saltwater. We tested if the explanation for this distribution pattern might be rooted in the ability to maintain escape performance across a range of salinities. Because guppies are prey to larger fish, escape performance is predicted to be highly correlated with survival and fitness. We subjected both species to a physical predatory stimulus to elicit an escape response at increasing levels of salinity (0-18 psu) and measured: 1) latency of response initiation, 2) distance traveled, 3) velocity, and 4) acceleration. We found the initial transition from freshwater to brackish water (6 psu) resulted in a decline in escape performance for both species, however P. picta was able to more rapidly acclimate and recover performance at higher salinities. These results suggest a decline in performance following a salinity increase and the inability to rapidly acclimate in P. reticulata might contribute to why this species is confined to freshwater in their natural environment.