During January 1977 three consecutive cold fronts crossed south Florida and the northern Bahamas which depressed shallow-water temperatures below the lethal limit for most reef corals. Digital thermal infrared data acquired by the NOAA-5 meteorological satellite, in situ water temperatures, and meteorological data were used to study the thermal evolution of Florida Bay and Bahama Bank waters. The third and most important frontal system depressed Florida Bay water below 16 degrees C, a thermal stress threshold for most reef corals, for 8 days. The minimum water temperature recorded in situ was 12.6 degrees C. Satellite data suggest that some Florida Bay coastal waters were at least 1 degrees C cooler than water at this site. Cold-water plumes (detected on satellite imagery) suggest that offshelf or offbank movement of cold, dense water follows bathymetry-controlled routes. Absence of viable shelf reefs opposite tidal passes supports this contention. Coral mortality at Dry Tortugas was up to 91 percent during the 1977 event. Coral and fish kills were also reported from other parts of the Florida Reef Tract and northern Bahamas. Study results show that cold-water stress conditions can exist over vast shallow-water areas and have residence times of several days. These observations suggest that aperiodic chilling processes have a limiting influence on reef community development throughout the Florida Reef Tract and northern Bahamas.