The impetus for this study was the mass bleaching event of giant clams in 1997–1998 at several reefs in the Great Barrier Reef (GBR, Australia). From September until December 1999, the study investigated the effects of high light intensities and increased temperature accompanied by a nutrient limitation on the metabolism of Tridacna gigas, to test, if these environmental changes can induce bleaching. In a 50-day trial similar conditions of the mass bleaching event were imitated stressing clams, collected from Orpheus Island and Nelly Bay, by a two- to threefold light intensity and an increased temperature of 4–6 °C. The objectives of the experiments were to determine whether high light intensities can induce changes in chlorophyll content or alter zooxanthella cell sizes and populations in the tissue. After 50 days of exposure to high light intensity the mantle tissue of stressed clams exhibited a decreased number of zooxanthellae per unit area from 19.8 ± 0.8 (× 107·cm–2) to 0.2 ± 0.2 (× 107·cm–2) (mean ± CL). Additionally, the average cell size of zooxanthellae were downsized from 7.4 ± 0.1 μm to 5.3 ± 0.1 μm (mean ± CL). Subsequently, the chlorophyll content of both, chl a and chl c1, declined as well, chl a from 192 ± 4 to 0.1 ± 0.1 μg·ml–1 and chl c1 from 145 ± 6 to 0 μg·ml–1 (mean ± CL). This study shows that increased light intensity and temperature are the main causes for bleaching in giant clams. Thus, the study confirmed the four major aspects involved in bleaching: (1) loss of symbiotic algae, (2) decrease of chl a/c1 in the remaining symbiotic algae, (3) retention of small zooxanthellae in the tissue and (4) release of ammonium (NH4+) into the water column while nutrient uptake of ammonium was largely blocked.