Coral colony survival depends greatly on the ability of their skeletal structures to minimize destruction caused by waves and currents. Hydrodynamic theory has been used to calculate the mechanical stresses operating on coral structures from currents, nonbreaking waves, and breakers. These calculations suggest two adaptive strategies are available to corals for minimizing hydraulically induced stress: (1) reduction in branch size and transformation of the corallum to massive hemispherical or encrusting form, and (2) development of oriented branched colonies. Strategy 1 is characteristic of Pocillopora, Montipora, Acropora, and the hydrozoan Millepora. Strategy 2 is characteristic of Acropora, particularly A. palmata. Measurement of hydr ulic conditions and colony morphology for A. palmata from Isaac Bay reef, St. Croix, supports our theoretical predictions regarding (1) distribution of branch azimuths; (2) angles of attack of branches, (3) colony eccentricity and alignment relative to flow, and (4) variation in branch shape and size. Hydraulically satisfactory morphology probably develops by a combination of a tropistic polypary growth response to prevalent flow conditions and a hydraulic selection process that destroys branches and colonies unsuited for the ambient hydraulic conditions.