The fall downstream migrations of juvenile American shad Alosa sapidissima and blueback herring Alosa aestivalis were monitored for 3 years in the Connecticut River at Holyoke Dam, Massachusetts. Decreasing water temperature—not increasing river flow—determined the time migration began and ended for both species. The migration of blueback herring began in September as water temperature declined to 21°C, peaked at 15–14°C, and ended in late October or early November at 10°C. The American shad migration began at 19°C, peaked at 14–9°C, and ended at 10–8°C. Most movement peaks were centered on quarter-moon periods; seven were centered on a new moon and none was centered on a full moon. The diel pattern of migration varied among the 3 years. In general, however, blueback herring were active throughout the 24-h period (peak activity at 1800 hours), and American shad moved in the afternoon and evening (peak activity between 1800 and 2200 hours). The migration of juvenile American shad may be mediated by decreasing temperature such that, as a threshold of 19°C is reached in the fall, the behavioral tendency to maintain position against the current in low light or at night is curtailed and the juveniles drift downstream. Sex ratios of American shad were not 1:1; females dominated in 1980 and males in 1981 and 1982. Sex ratios of blueback herring were 1:1 in 1980 and 1981, but males significantly outnumbered females in 1982. The mean total fish lengths increased significantly during the course of the migration for American shad in 1982 and for blueback herring in 1981.