The study was conducted at the fringing reef of Sesoko Island (Okinawa, Japan), in the north-western Pacific. Artificial injuries were inflicted to the upper portions of colonies of the massive coral Porites lutea. The colonies were fixed to cement blocks and placed at two reef locations of different light intensity: one with moderate light (20–30% of incident photosynthetic active radiation, PAR0) and one with low light (2–5% PAR0). After six months of the treatment, all injuries were photographed. Analysis of photographs made under light and electron microscopes showed that the coral was able to recover from the injuries at both light variants. Under moderate light, some of the injuries completely healed and some injuries recovered partially. Under low light all injuries recovered partially. These partially recovered injuries were occupied by algae at dead portions of the injuries. In moderate light, coral polyps overgrew all algal settlers and were the winners in the competition for substratum. In low light, Peyssonnelia conchicola, Corallophila apiculata, Centroceras clavulatum, Anortichium tenue, and Lobophora variegata that colonized the injured areas overgrew coral polyps and were the winners in the competitive struggle for the substratum. It was shown that overgrowing is the main mechanism of coral–algal competition for substratum in light ranging from 2 to 30% PAR0. This mechanism of overgrowth of corals by algae, and the reverse, are discussed in detail.