Laboratory studies assessed the effects of gradual and rapid decreases in temperature on 9-14-cm long threadfin shad collected from a tributary of the Tennessee River. Fish acclimated to 15 C and exposed to temperatures cooling 1 C each 72 hours decreased in feeding activity and schooled less compactly beginning at 9 C. At lower temperatures they lost orientation and were insensitive to external stimuli, and none survived exposure to 4 C. Threadfin shad exposed to sudden cooling (2 to 5 C decreases in 4 hours) lost equilibrium in proportion to the magnitude of temperature decrease if minima of 4 to 6 C were reached. Cold-shocked fish that had lost equilibrium for 0.2, 30, and 90 minutes before being returned to warmer water experienced mortalities of 32, 79, and 94%, respectively.