The dynamics and energetics of subpopulations of a numerically dominant deposit—feeding polychaete species, Pectinaria californiensis Hartman, were studied and compared with crude determinations of the same for the larger but rarer coexisting species of the same feeding mode, the heart urchin Brisaster latifrons (Agassiz) and the sea cucumber Molpadia intermedia (Ludwig). Monthly samples, taken for 1 yr at five stations in Puget Sound representing different habitats, were used in conjunction with laboratory measurements of respiration to assess the effects of seasonal and spatial variation in growth, mortality, and respiration on estimates of energy flow through these species populations. Pectinaria larval settlement (2,900—24,000 animals/m2) occurred at all locations in June 1970. Two of three age classes or cohorts were present simultaneously. Pectinaria represented 4%—26% of macrofaunal (>1 mm) biomass, and 9%—47% of numbers at the five locations, based on the mean of four seasonal estimates. At the two stations where Brisaster and Molpadia coexisted with Pectinaria they contributed, respectively, 79% and 4% of macrofaunal biomass at one station and 13% and 63% at the other. Recruitment and growth of the two echinoderms appeared negligible, as neither numbers nor mean size changed during the study period. Annual production of Pectinaria, not including excretion or mucus production, varied 1.4—48 g C/m2°yr (14—49 kcal/m2°yr). The ratio of annual production to mean annual biomass, varying in the study area 3.3—5.5m provided a better estimate of turnover than the more commonly used ratio based on the lifetime of a cohort because of the difficulty of determining lifespan, a problem with most long—lived organisms. Pectinaria contributed 14%—42% of macrofaunal respiration in the area studied. But these numbers were shown to be affected by the failure to reproduce in the laboratory in situ oxygen—tension conditions. Such overestimates of population respiration from laboratory measurements were most marked for Brisaster and Molpadia. These latter estimates, while reflecting biomass data, unrealistically overshadowed the respiration of all other organisms. The sum of Pectinaria production and respiration (corrected for in situ oxygen tension) varied 2.6—9.2 g C/m2°yr (27—98 kcal/m2°yr), reflecting differences in rates of growth and mortality among stations. At two stations where primary production data were available, Pectinaria assimilated at least 1.3% and 3.3% of the carbon fixed by phytoplankton. Subsequently, 0.6% and 1.7%, respectively, were made available to predators and decomposers in the form of Pectinaria flesh. Because of its greater turnover of assimilated energy, Pectinaria contributed more to metabolic processes and to foodchain dynamics of the seabed than did the coexisting echinoderms. The echinoderms, on the other hand, may exert important influences on the structure of the community not accounted for in normal energetic assessments. Spatial and temporal variations in energy flow through species subpopulations can be large, and thus may limit the usefulness of a stability assumption in the development of predictive models for organic matter budgets.