Blooms of Noctiluca scintillans are reported for the first time in the Red Sea offthe south-western coasts of Saudi Arabia. During the present study, surface water samples werecollected weekly on the coasts of the Al Shuqayq region from February to April 2004-2006. The abundance of N. scintillans correlated negatively with most nutrients,as well as the cell densities of diatoms and dinoflagellate species. Microscopic examination of live cells from Noctiluca blooms showed the presence of some species of diatoms and dinoflagellates within the Noctiluca body - confirmation of its grazing on these microalgae algae. Thepresence of a Noctiluca bloom in the coastal waters off south-western Saudi Arabia could be linked indirectly to water eutrophication by an increase in prey abundance.The physico-chemical properties of Red Sea coastal waters should therefore be monitored regularly in order to minimize the formation of harmful algal blooms, which may affect all food web levels, including the human level.