We conducted swimming performance tests on native and nonnative fishes commonly found in Arizona streams to evaluate the extent of differences in swimming ability among species. Fishes with similar mean lengths were subjected to stepwise increases in water velocity in a laboratory swim tunnel until fish could no longer maintain position. Nonnative fathead minnows Pimephales promelas and red shiners Cyprinella lutrensis exhibited swimming abilities similar to native longfin dace Agosia chrysogaster, speckled dace Rhinichthys osculus and spikedace Meda fulgida. Nonnative mosquitofish Gambusia affinis exhibited swimming ability similar to native Gila topminnows Poeciliopsis occidentalis. Desert suckers Catostomus clarki, bluehead suckers Catostomus discobolus and speckled dace exhibited behavioral responses to high water velocities that may confer energetic advantages in swift water. Differences in swimming ability do not appear to adequately explain the disproportionate removal of nonnative fishes via flooding. Behavioral responses to high flows are more likely the mechanism that allows native fish to persist in streams during flood events.