ABSTRACT: Drift algae are an important food resource for many intertidal marine invertebrates. This study quantifies the arrival rate of drift kelp on the intertidal zone of central Chile and analyses its nutritional role in maintaining populations of thesea urchin Tetrapygus niger. The analysis was based on stomach contents and on stable isotope ratios of the food and the sea urchin. Gonad development was also evaluated. Lessonia nigrescens was the kelp most commonly observed, and itsarrival pattern showed both temporal and spatial variation. The highest levels of drift kelp occurred in late spring, early summer and part of the autumn. Lessonia spp. were the most important food of T. niger, comprising 68% of the diet,and most of the assimilated carbon came from this source. The availability of drift kelp influenced gonad development. Trophic subsidies such as drift kelp are clearly important to intertidal animals of temperate ecosystems and have effects at differentlevels of biological organization. Two alternative scenarios of community organization are proposed for tidepools, depending on drift kelp availability.