Sexual reproduction of the Red Sea coral Stylophora pistillata was followed at Eilat in a long-term study (1974-1984) .Field examination of over 9000 colonies through 119 months indicated that S. pistillata had a reproductive season of approximately 8 months (varying from 6 to 9 months). Premature planulae and eggs were aborted following winter storms, resulting in a lowering of the planular index and the number of female gonads per polyp. Histological examinations of tissue from 20 large colonies which were studied for several years, until they were found dead in situ, indicated that either sexuality (reproductive states) and/or fecundity could be completely altered from one reproductive season to the next: i.e., hermaphroditic colonies exhibiting high fecundity in one season became male or even sterile thereafter, and vice versa. In addition, great variability in reproduction between successive years was recorded in sexuality and in the fecundity of shallow water populations. Shallow water colonies (5 m) possessed up to 5 times more female gonads per polyp and shed 5 to 20 times more planulae than deep water colonies (25 to 45 m) in which the reproductive season is 2 to 3 months shorter. We suggest that the changes in the hermaphroditic, male, or sterile modes of re production in S. pistillata are from energy limitations and stress conditions. Since reproductive activity probably involves significant energetic expenditures, any stress or diminution in energy resources affects sexuality or fecundity. This should be con side red before formulating any general hypothesis on coral reproduction.