In Hawaii, zooxanthellate corals of the genus Leptoseris have been observed growing in situ to depth over 150 m (Kahng et al. 2010). Due to clear oligotrophic waters and geological history, the main Hawaiian Islands have more potential area for zooxanthellate coral habitat at mesophotic depths than in shallow water <30 m (Locker et al. 2010). In the Indo-Pacific, Leptoseris spp. are among the most abundant zooxanthellate corals in the lower photic zone (Kahng et al. 2010). However, little is known about the life history traits of these dominant, deep-water corals. While slow metabolism and growth is a common strategy for survival in an energy-limited environment, whether deep-water zooxanthellate corals adopt this strategy has not been previously studied. In October of 2006 and again in November of 2009, the same colony of Leptoseris hawaiiensis at 90 m in the Au’au Channel, Hawaii was measured with the aid of calibrated parallel lasers from the Hawaii Undersea Research Laboratory’s Pisces IV submersible (Figure 1). The conserved spatial pattern of corallites on the colony (skeletal growth is outward but not upward) was used to verify measurements and account for parallax. Diameter of the colony increased 6.5 +/- 1.0 cm in 37 months. The average radial extension rate was 1.1 +/- 0.3 cm per year which is comparable to some shallow-water corals. Efficient light harvesting mechanisms may facilitate the growth of these specialized zooxanthellate corals under the low light conditions at depth (Kahng et al. 2012). Despite their moderate rates of areal expansion, L. hawaiiensis and other agariciid species at these depths are characterized by thin skeletons (2 – 5 mm in thickness) and therefore have relatively slow rates of calcification compared to shallow-water corals. During these surveys colonies exceeding one meter in diameter were regularly observed at 80 – 100 m depths. Assuming similar average growth rates ages of these large plate corals (Figure 2) can be conservatively estimated at ~50 years old. The longevity of these fragile colonies confirms the lack of natural disturbance associated with mesophotic coral ecosystems (Bongaerts et al. 2010).