Like plants, sessile invertebrates are often morphologically modified at high densities. At high densities acorn barnacles commonly form hummocks of tall, densely packed individuals. We examined hummock development and its consequences on filter feeding and growth in the northern acorn barnacle, Semibalanus balanoides. Hummocking occurs in response to high recruitment densities and growth rates, which intensify competition for primary substrate space. In the field, hummocks are more common at low than at high tidal heights, paralleling within-site variation in recruitment and growth rates. Hummocks also are most pronounced at high-flow sites with high barnacle recruitment and growth rates. In laboratory flume studies, particle capture rates were higher for individuals near the peak of hummocks than for solitary individuals, and were lower for individuals in the troughs between hummocks than for solitary individuals. Hummocked individuals are elevated above the surface and are thus likely exposed to higher particle fluxes than either individuals between hummocks or solitary individuals. These patterns in particle capture closely match patterns of barnacle shell growth in the field. The shells of hummocked individuals were larger than those of solitary individuals, which were larger than the shells of individuals in troughs between hummocks. The tissue growth of solitary individuals, however, is lower than that of crowded individuals, apparently since, without neighbors, solitary barnacles must allocate more resources to structural support than do crowded individuals. Thus, crowding may benefit individual barnacles by reducing their skeletal support costs. Most studies of crowding in sessile, space-limited invertebrates have focused on negative, competitive effects. Our results, along with previous work showing that crowding may benefit northern acorn barnacles by buffering them from heat and desiccation stress and increasing reproductive output, illustrate that crowding also can positively affect sessile organisms. We suggest that interactions among most sessile, space-limited invertebrates are best viewed as a balance between negative and positive effects.