Young and adult southern flounderParalichthys lethostigmawere subjected to decreasing temperature (1°C every 24 h) in fresh and salt water in the laboratory to study their lower temperature tolerances. Survival of adult southern flounder overwintering in fresh- and saltwater ponds was also observed. Young and adults in the laboratory were more tolerant of cold in salt water than in fresh water. Sixty percent of young fish died in fresh water, but only 10% in salt water, as temperatures declined to 2°C; all adults died in fresh water and 63% died in salt water after temperature declined to 1°C. Young died sooner than adults in fresh water, but later in salt water, during temperature decreases. Mortalities continued after water rewarmed. Some young and all adults ceased feeding when temperatures dropped below 6.4°C. In ponds, a minimum temperature of 8°C in fresh and salt water resulted in 14% mortality of adults, whereas a 5°C minimum temperature in a freshwater pond caused 100% mortality. Temperatures below about 10°C in fresh water and 7°C in salt water are judged to put southern flounder at risk of cold-induced death.