Branching hermatypic corals, Acropora cervicornis (Lamarck) were transplanted into two areas of the Florida reef tract where they do not normally grow. The growth-rate of these corals and a control group on a thriving reef was measured twelve times between December 1960 and February 1962. Average growth-rate of the transplanted corals was less than one half that of the control group, which grew at 10 cm per year. Growth-rates, however, showed considerable variation. During a 2-month period one transplanted group grew as fast as the control group, but after 10 months died when the water temperature dropped to 13.3 degrees C. Seasonal fluctuations in growth at all stations could be correlated in a general way to temperature fluctuations . . . Growth was greatest between 28 degrees and 30 degrees C. The results suggest that the method may be used as a standard ecological tool for determining growth tolerances of other reef-building organisms.