Benthic marine invertebrates recruit unevenly. During dense settlement years, cannibalism may contribute significantly to juvenile mortality but may also function as a constructive process by promoting the growth of individuals who cannibalize. As early as 4 days post-metamorphosis we observed frequent cannibalism among juveniles of the sea star Asterias forbesi. To determine if cannibalism was density dependent, we placed 2-3 week old juvenile sea stars at varying densities in 200 mL beakers. After one week, mortality rates of isolated juveniles were 10% while mortality rates at densities of 2, 10 and 20 juveniles per beaker were 10%, 73% and 90% respectively. We then conducted trials with juvenile densities matched to field observations made during moderate recruitment years. Control juveniles experienced 5 ± 3.2% mortality, while juveniles allowed to cannibalize each other experienced 20 ± 2.7% mortality. We then created pairs of juveniles with differences in disk diameters ranging from 25-775 µm. We found that cannibalism was size specific: larger juveniles were more likely to be cannibalistic than smaller juveniles. We also found that the greater the size difference between juvenile pairs, the more likely they were to cannibalize. Finally, we found that cannibalism occurred even when juvenile mussels were provided as an alternative food source, suggesting that cannibalistic behavior is not limited to years when other food sources are limiting. To our knowledge these are the first experiments showing that juvenile cannibalism in A. forbesi is density dependent, size specific, and occurs at natural densities.