Photophase and illumination affect many fish activities. In this study, we examined their effects on the critical swimming velocities (Ucrit), swimming gait patterns, and oral grasping behavior of five California estuarine fishes. All species (4-5 cm SL) swam similarly (mean U-crit range: 30-36 cm/sec) under day/light conditions. However, both nighttime photophase and darkness decreased Delta Smelt (Hypomesus transpacificus) swimming velocities. Congeneric Wakasagi (Hypomesm nipponensis) swimming performance also decreased at night/dark conditions. Regardless of photophase and illumination, Delta Smelt, Wakasagi, and Splittail (Pogonichthys macrolepidotus) exhibited three swimming gaits: intermittent stroke-and-glide at low velocities, continuous stroking at moderate velocities, and intermittent burst-and-glide at high velocities near Ucrit. In contrast, Chinook Salmon (Onchorhynchus tshawytscha) used only two swimming gaits: continuous stroking and burst-and-glide under all conditions. Inland Silversides (Menidia beryllina) used these two gaits under light conditions and all three gaits under dark conditions. Some Wakasagi, Splittail, and Chinook Salmon orally grasped the upstream screen in the flume at moderate to high water velocities. Oral grasping does not require jaw teeth and may represent adaptive behavior in natural habitats. Regarding vulnerability to water diversions that operate in the dark at night, the threatened Delta Smelt and introduced Wakasagi, comparatively, may be more at risk than the other species.