The sea pen Ptilosarcus gurneyi Gray provides a major source of food for at least seven predator species in Puget Sound: the asteroids Hippasteria spinosa, Dermasterias imbricata, Crossaster papposus, and Mediaster aequalis, and the nudibranchs Armina californica, Tritonia festiva, and Hermissenda crassicornis. Ptilosarcus is long—lived (15 yr +), takes 5 or 6 yr to reach sexual maturity, and has a spatially clumped pattern of recruitment. It never grows large enough to avoid predation by four of its predators, and the refuges and escape mechanisms of Ptilosarcus are not sufficient to explain its abundance in the face of this predation. Determination of rates of critical life history processes of species involved in this association, e.g., recruitment, growth, and predation, permitted estimates of the effects of the activities of each species on the others in the association. Particular attention was paid to age structure and to the natural history during the periods shortly following metamorphosis of each species. In dense recruitment patches, 97% of Ptilosarcus mortality occurs during the 1st year because of predation by Hermissenda, Tritonia, and Crossaster. Adult pens are eaten by Hippasteria, Dermasterias, Mediaster, and Armina, which together remove 2 pens/m2°yr, or 3.1% from the 10 adult year classes combined. The latter four predator species selectively prey upon the largest pens available. The time required for capture and consumption increases at a slower rate with prey size than does prey biomass. Recruitment of the long—lived food specialist Hippasteria seems to be impaired by the spatially unpredictable and clumped nature of Ptilosarcus recruitment. Although the adults have an opulent food supply, the young Hippasteria require small Ptilosarcus to feed on and rarely find enough. The generalists, which are not dependent upon one prey during their recruitment and are able to maintain themselves without Ptilosarcus, could in theory increase until they have reduced the sea pen populations to below the level needed by the specialists and thus outcompete them. However, predation on the generalists by a higher order predator, the asteroid Solaster dawsoni, removes each year at least 9.4% of the adult Mediaster and 38% of the adult Crossaster, a rate at least as great as, and usually greater than, the rate of recruitment into the populations of adult prey. The characteristic organization of the Ptilosarcus association and the existence of Hippasteria may depend upon S. dawsoni.