The red sea urchin Loxechinus albus, an economically important species oceurring along the Chilean littoral benthic systems, has been heavily exploited by artisanal fisheries in recent decades. However, basic knowledge of its trophic biology is still scarce. Studies of this kind are critical to the successful development of farming techniques as an alternative to the harvest of natural populations. The aims of this study were to quantify the composition of L. albus' natural diet, to determine the assimilation efficieney of the most important dietary components, and to experimentally examine the species' trophic selectivity. Adult and juvenile specimens were sampled in spring 1993 and summer, fall and winter 1994 from the shallow subtidal zone of Iquique, northern Chile. Neither juveniles ñor adults showed significant seasonal differences in dietary composition. The main Ítems in the diet were the green alga Ulva sp. for juveniles and the kelp Lessonia sp. for adults. In adults, assimilation efficieney (as percentage of the total organic matter ingested) was not significantly different among individuals that consumed Lessonia sp. (48.7 %), Ulva sp. (44.6 %) and Macrocystis (38.1 %), whereas in juveniles assimilation efficieney was significantly higher for individuals fed on the chlorophyte Ulva sp. (55.4 %), followed by Lessonia sp. (35.0 %) and Macrocystis (25.5 %). These results suggest that L. albus undergoes an ontogenetic shift in the diet, consisting of a differential foraging strategy between juveniles and adults.