Sympatric stream-dwelling river chub (Nocomis micropogon) and smallmouth bass (Micropterus dolomieu) were video-taped swimming in the presence of single cylinders spanning a flume. Fish were acclimated and tested at 13, 18 and 23 degreesC swimming in an increasing-velocity test with single vertically or horizontally oriented cylinders 6.4, 13, 19 or 25 mm in diameter. Cylinder orientation had little effect on entrainment. Both species avoided cylinders at low speeds. At intermediate speeds, some fish entrained on the cylinders, but they were displaced at higher speeds. Entrained fish made no regular swimming motions, but the median and paired fins and the body moved continuously and irregularly. The periodicity of body perturbations decreased and the amplitude increased with current speed. These also increased with cylinder diameter, but the variability was high and differences were not significant. Chub entrained in larger numbers and for longer over a wider range of current speeds and temperatures than bass. Entrainment by chub increased with cylinder diameter up to 13-19 mm, but declined for 25 mm cylinders. Bass did not entrain on 6.4 mm and 25 mm cylinders and showed very little entrainment at 13 and 18 degreesC, but entrainment was similar to that of chub at 23 degreesC.