A high-density population of the polychaete Melinna palmata Grube was the focus of a two-year study at a shallow-water location in Inner Galway Bay on the west coast of Ireland. Mean densities were highest in the second year of the study following successful recruitment. Size-frequency analysis of population structure (April 1983 to March 1985) revealed the presence of four cohorts, with the 1983 settlement becoming the dominant modal class. Size comparison of this cohort with the 1982 settlement showed that growth rates were lower and mortality higher in the 1983 settlement, which may be related to density dependent intra-specific competition. M. palmata at this location live for 2 to 2.5 yr with a small number surviving to 3 yr of age. M. palmata in Galway Bay is dioecious and has an even sex ratio. Reproductively mature worms were approximately 2 yr old when they first spawned. Spawning took place between May and July, when mature oocytes had a modal size of ca. 290-mu-m. Proliferation of previtellogenic oocytes (ca. 10-mu-m) from the gonads occurred following spawning. Growth over the winter was slow and was followed by a period of rapid growth between March and May, coincident with rising water temperatures. The presence of mature unspawned oocytes, which were resorbed after commencement of new proliferation, was taken as evidence of the species propensity for polytely. However, post-spawning mortality appeared to be high, so that only a small number of individuals survived to spawn the following year. The highest abundance of mature males was encountered between May and August, but lower levels of mature stages were present throughout the year.