Observations are presented of the effects of winter water conditions on the survival, behavior and color patterns of 15 species of marine fishes, comprising 11 families and 80 specimens held in aquaria with continuous flowing water. Only weakfish were found to float when dead, all others remained on the bottom of the test aquaria. The behavior patterns exhibited prior to death were as varied as the species studied. These observations may help to explain aggregations of dead fishes that are often detected accidentally during winter sampling. The largest kill of the study specimens occurred during the 15-20 December, 1962, cold snap. Field observations froin Georgia to Texas reported kills of several of the same species. Schwartz (1963) pointed out the need for observations on the effects of low temperature waters on the behavior and survival of marine fishes as a prelude to speculations regarding natural calamities. This paper gives further observations and presents data on 15 species of marine fishes, comprising tt families and 80 specimens, which were subjected to natural winter water temperatures and salinities. Specimens were obtained from pound nets near the mouth of the Potomac River, Smith Point, Virginia, on 25 September, 1962. A boat, equipped with two circular tanks, six feet in diameter and three feet deep, which were supplied with Chesapeake Bay surface water, was used to transport the fish the 20 miles to the Chesapeake Biological Laboratory. Temperature and salinity conditions at the time of capture at Smith Point were 20.7 C and 19.5 ppt. The fish were transferred into four-foot-square indoor aquaria, with unfiltered, flowing water (20.2 C and 16.1 ppt) pumped from the Patuxent River, Solomons, Maryland. Distribution of the fish in the aquaria was as follows: Tank 1, cownose ray, southern stingray and sharksucker; Tank 2, kingfish, croaker, red drum and stargazer; Tank 3, spot, filefish, striped burrfish and weakfish; Tank 4, black sea bass, spot (small), searobin and swellfish; and Tank 5, toadfish. Beaven (1960) reported the general salinity and temperature conditions observed at Solomnons over a 20-year period. Salinities during the observation period and their relationship to 25-year averages are illustrated in Figure t. The oxygen conditions of the water in the aquaria were always more than adequate for fish survival. Generally, the fall and winter of 1962-63 was colder than usual. The Chesapeake Bay to Point Lookout was frozen completely over on 18 February, 1963, for the first time since 1934. Ice, 6-10 inches thick, prevailed from the head of the Bay to James Island (62 nautical miles). A one-quarter to one-inch thickness of ice covered the Bay southward another 20 nautical miles to Point Lookout. 1 Contr. No. 246. Natural Resources Institute of the University of Maryland, Solomons, Maryland.