Bioerosion by boring organisms is one of the major destructive forces operating on reef. The aim of this study was to investigate the bioerosion by microflora of a scleractinian finger coral Acropora humilis, which collected from the reef edge of El Ain El Sukhna (Gulf of Suez- Red Sea) by using the Scanning Electron Microscope (SEM). The collected colonies of A. humilis were solid, very porous, and branching. These colonies have two colors; brown color with purple branch tips and yellow color with cream branch tips. Individual branches form fat fingers; 10 to 25 mm in diameter and less than 200 mm in length, tapering to large dome-shaped axial corallites. Small branchlets or incipient axial corallites usually occur at the base of main branches. Radial corallites are cup-shaped and have two sizes, the larger are usually in rows and have thick walls and only slightly increase in size down the sides of branches. Axial and radial corallites have a series of vertical rods arranged in concentric rings and horizontal radial and tangential bars. The radial bars form the sclerosepta along with the vertical rods. The tangential bars are synapticulae that connect adjacent sclerosepta to one another. Series of fasciculi form the characteristic scale-like appearance of A. humilis skeleton. The bioerosion was investigated at least in one branch of some colonies which were harbored by fungi, green algae and cyanobacteria; led to loss of tissues and erosion of rods, bars and fusiform crystals. Acropora humilis is subjected to bioerosion due to: its surface which covered by muco-polysaccharides, its high porosity and its branching form; whose facilitate colonization by boring organisms.
Key words: Scleractinian Coral, Acropora humilis, Skeleton, Crystals, Boring Microflora, Bioerosion, Scanning Electron Microscopy.