The talitrid amphipod Orchestoidea tuberculata (Nicolet) is one of the dominant species inhabiting the upper intertidal of a variety of morphodynamic types of exposed sandy beaches of the Chilean coast. The macroalgae stranded on the beach are the primary food source for this amphipod. In this study, we analyzed the influence of the availability of macroalgae and beach morphodynamic features on the population abundance and life history of O. tuberculata. Eleven beaches of the coast of Valdivia (ca. 39 degrees S) were sampled, during a period of 13 months, to determine biomass of stranded macroalgae and beach characteristics. Seasonal faunal samples were collected to characterize the populations of O. tuberculata at each site. The results showed the configuration of two beach groups with significant differences in biomass of stranded macroalgae, independent of their morphodynamic features. The total abundance of amphipods, the abundance of females, ovigerous females, males and juveniles, as well as body size, were significantly higher at beaches with higher biomass of stranded macroalgae. Fecundity did not differ significantly between both beach groups, although the expected number of eggs per female was nine times higher at those beaches with the highest biomass of stranded macroalgae. These results suggest that food availability has a significant role on the population abundances and life history of O. tuberculata, independent of the morphodynamic type of sandy beaches, situation which departs from paradigmatic models in sandy beach ecology.