Population dynamics of the intertidal mussel population in the River Exe estuary, Devon (UK) over 7 yr (1976 to 1983) are described. There was no significant trend in adult numbers, and annual variations were small (1.5-fold, minimum to maximum). In contrast, the density of 'spat' mussels (0-year) varied uldely ( l ?~) , suggesting that a strong damping process acted during the first year. Considering just first-year mortality, failure to settle on the beds (k3) had the greatest influence on annual variations in total mortality (K). But losses in the first winter after settlement (k5) were strongly and positively denslty dependent. k3 was inversely density dependent, but only in 6 out of 7 yr. Other first-year losses were independent of density. After the large first-winter mortality (mean 68 %), there were only small gains in summer and small losses in winter for the next 4 yr. Thereafter, increasingly large losses occurred (mean 39 % ) each summer after spawning and each winter (mean 24 ?h), due mainly to bud predation. Some of the over-summer and over-winter changes in adult densities were density dependent, but their effect was small and only served as a 'fine tuning' mechanism keeping densities within very narrow limits. Though mortalities at various stages were correlated with the weather, the main feature of the population was its pronounced stability, due mainly to the powerful regulation acting during the first year on the mussel beds.