Lottia gigantea, a large (up to 8 cm in length) limpet of the California and northern Mexican coast, lives in association with an approximately 1,000 cm2area of algal film in which its grazing marks can be seen, whereas the remainder of the rock surface is usually free of any visible film. These areas of algal film represent the territories of the Lottia; within them the animals do all their grazing. They keep their territories free of other organisms by shoving off any intruders: other Lottia, grazing limpets of the genus Acmaea, predatory snails, and sessile organisms such as anemones and barnacles. Within 2—3 weeks after Lottia were removed from their territories, the density of Acmaea in these territories increases to that found outside them, and the algal film disappeared. Apparently the defense of the territories against intrusions by other grazers permits the growth of a thick algal film on which the Lottia can effectively graze. Their reaction to sessile organisms prevents these animals from encroaching on and covering the territory. Their response to predatory snails may be a defense mechanisms.