Critical swimming speeds (CSS) for groups of menhaden were determined in an array of time and water velocity combinations. Critical swimming speeds varied between 20.7 body lengths/s (BL/s) for short time intervals (2 min) to 15.8 BL/s for long time intervals (64 min). The experiments were repeated with the same group of fish to assess the effect of previous experience, and mean CSS increased by 2.3 BL/s. Tailbeat frequency increased steadily with increasing velocity. Differences in tailbeat frequency occurred among fish in different positions in the groups. Time to exhaustion for menhaden in groups of one to eight fish swimming at constant velocities were determined to assess the effect of group size on swimming performance. Time to exhaustion increased by a factor of 10 as group size increased from one to six fish.