Data from quantitative samples of the benthos at a 200-m site in central Puget Sound, collected twice yearly in most years between 1963 and 1992, were evaluated to determine the extent to which species composition in a continental-shelf depth community exhibits long-term persistence. Study results showed that the most abundant species were consistently present over the 30-year period. However, measures of species composition (e.g., similarity, diversity) reveal a subtle, gradual change in the community over time. Among the changes are (1) multi-year periods of greatly increased abundance of the common species; (2) an overall increase in the total abundance of the benthic community beginning in the mid-1970s; (3) periods of increased abundance, during the late 1970s and early 1980s, of two species that are tolerant of organic enrichment; and (4) the steady decline in abundance of the large burrowing echinoderm, Brisaster latifrons as a consequence of the lack of recruitment to the site since 1970. Despite the conspicuousness of these changes, there are no observed environmental factors that readily explain them. Circumstantial evidence suggests that climate-related change in Puget Sound circulation beginning in the mid-1970s, organic enrichment associated with a nearby large source of primary-treated sewage, and the influence of changes in the abundance of the large echinoderms on the smaller species are potential agents of change. The principle reasons for our inability to identify causes of long-term change in the Puget Sound benthos are (a) inconsistent long-term monitoring of environmental variables, (b) the lack of quantitative information about long-term changes in plankton and fish populations, (c) lack of knowledge of specific predator/prey and competitive interactions in soft bottom benthos, (d) unknown influence of moderate levels of contamination on biota; and (e) lack of understanding of possible linkages between climate regime shifts and fluctuations in local biological populations.