- We investigated the abundance and distribution of calcium oxalate crystals in the leaves of wild populations of a Negev desert lilyPancratium sickenbergeriin relation to herbivory. Three species of herbivores are known to eat the leaves of this lily: a small antelope, the dorcas gazelle Gazella dorcasa moth larva Polytella cliensand a land snail Eremina desertorum. All three species eat only those parts of the leaves where calcium oxalate raphides are absent, suggesting that it is an effective defensive chemical. We compared the abundance of raphides in three isolated lily populations that differed only in the amount of gazelle herbivory. Within lily populations, we found neither size-related differences in raphide abundance nor differences in raphide abundance between plants that had previously been partially consumed and those that had not. We found significant differences among lily populations in the amount of calcium oxalate crystals in their leaves, with the most raphides being found in the population suffering most herbivory, fewer in a population with intermediate herbivory, and the least in a population without gazelle herbivory. Additionally, sand samples showed no differences among populations in two major nutrients (nitrogen and phosphorus) but significantly more calcium in the sand in the population without herbivory. Thus, calcium oxalate abundance in the leaves of Pancratium sickenbergeri is not constrained by resource availability but rather appears to have been selected for by gazelle herbivory. This is the first study to show the effects of selection on calcium oxalate production in a wild plant by a wild herbivore.