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Structure and injuries of octocoral communities (Octocorallia: Alcyonacea) of Ecological Reserve Siboney-Juticí, Santiago de Cuba, Cuba. In the spur-and-groove reefs of the Ecological Reserve Siboney-Juticí (Southeast Cuba) octocorals are one of the predominant components of the sessile fauna. Main objectives of the present paper are characterizing the composition, structure and conservation status of the octocoral communities and assessing on the prevailing environmental conditions in the study area. For data collection, six sampling sites were located every 2-3km ranging from 12m to 17m depth along the spur-and-groove reefs. In each site, 22-26 1m 2 quadrants were zigzag arranged every 2m. The density of colonies was determined per site. The severity degree and predictability of environmental conditions were inferred by using the Heterogeneity and Equitability indexes, respectively. Hydrodynamic stress and the Comparative Pollution Index were also assessed. Current affectations of the octocoral communities were also determined and classified into four main categories: mechanical damage, diseases, predation and invertebrate and macroalgae overgrowth. As a result, 25 species represented by 752 colonies were recorded and Eunicea flexuosa and Gorgonia ventalina were the most abundant. The occurrence of Eunicea succinea forma succinea constituted the first report for Eastern Cuba. Site densities ranged from 3.58±1.84 to 7.58±2.16 colonies/m 2 and considered from moderate to low. The biggest densities were reported at both sides of the San Juan River mouth. Despite of the composition and structure of the octocoral communities, low to high hydrodynamic stress and low and moderate levels of contamination were inferred, it is likely that these indexes could have been overestimated due to the sensitivity of the indicator species to other factors such as sedimentation. The environmental conditions were mostly favorable and stable. In general, the number of dead colonies was low and mostly caused by the detachment of the substrate and overgrowth of Millepora alcicornis. Injuries were mainly brought about by mechanical damage, followed by coenenchyme loss, predation by Cyphoma gibbosum and macroalgae overgrowth. On the other hand, negative effects by Hermodice carunculata predation, bleaching, Red Band and Black Band Disease were not observed. Among sampled species, G. ventalina colonies were the most severely damaged within the community. As a whole, octocoral communities of spur-and-groove reefs in the Ecological Reserve Siboney-Juticí showed a good conservation status. Rev. Biol. Trop. 58 (4): 1211-1221. Epub 2010 December 01.