We documented the physical habitat characteristics and fish diversity of a hyperthermal rockpool on Loggerhead Key in the Dry Tortugas National Park during July 2000. Rockpool temperatures ranged from 30.5 °C to 35.8 °C and oxygen varied from 6.4 to 3.5 mg/L depending on depth and time of day. Seven fish species from five families inhabited the rockpool. Critical thermal maxima (CTMax) and critical oxygen minima (COM) were measured for three species. French grunt Haemulon flavolineatum was the most temperature tolerant fish (CTMax = 37.9 °C), followed by cocoa damselfish Pomacentrus variabilis (36.1 °C), and white mullet Mugil curema (35.0 °C), respectively. Cocoa damselfish were more tolerant of hypoxic conditions (COM = 0.8 mg/L) than either French grunt (1.2 mg/L) or white mullet (1.5 mg/L). French grunt and cocoa damselfish resorted to aquatic surface respiration at respective dissolved oxygen levels of 2.6 and 1.7 mg/L, whereas white mullet did not display this behavior at oxygen concentrations as low as 1.5 mg/L. High-temperature and low-oxygen responses of the three species were not exceptional, suggesting that behavior and not physiology is the major factor allowing Loggerhead Key fishes to exploit hyperthermal habitats.