Detailed plankton sampling in the Gulf of Elat, Red Sea, demonstrates a lunar periodicity in the abundance of the foraminifer Globigerinoides sacculifer in surface water as well as below the photic zone. Shell size distribution coupled with abundance changes and laboratory observations suggests the following scheme for the life cycle of this spinose foraminifer: Young individuals of roughly 200 µm (average diameter) are found in surface water 7–8 days after the full moon. During the course of 1 week they grow rapidly to an average size of 300–400 µm. The larger and mature individuals, many of which having a saclike chamber, start to sink down below the photic zone. They add a final chamber, shed their spines, thicken their shell, continue to sink well within the main thermocline, digest their symbionts, and eventually reproduce via gametogenesis at the full moon. We do not have information about the fusion of the gametes, the embryonic growth stages, and the vertical migration of the young foraminifera. However, 1 week after the full moon, small individuals appear again in surface waters and the cycle repeats itself. Although the mechanism by which the moon is synchronizing the reproduction cycle is not clear, it obviously offers better chances for the gametes to fuse. Sinking to deeper water may provide the following advantages: avoidance of predators at the crucial stage of their life cycle when they are without their spine protection; synchronization of reproduction by triggering gametogenesis due to the change from light/dark cycle to complete darkness; better chances for the gametes to concentrate and fuse on a narrow sigma-t surface at the zone where the pycnocline is steeper; and finally, better chances for survival for the young unprotected foraminifera who start their growth in an environment that contains fewer potential predators.