The reproductive biology and population ecology of the sabellid polychaete, Sabella pavonina (Savigny) inhabiting an intertidal sand bank in Langstone Harbour, Portsmouth, was investigated between February 2007 and June 2009. ‘Blind’ thrown quadrats and sediment cores were used to characterise population distribution patterns at the site, and monthly coelomic fluid samples from 30 individuals were used to investigate the reproductive biology of the population, sex of the specimen, the presence of coelomocytes and the development of oocytes. Highest population densities (30.58 m−2 ± 8.05) were recorded on the low shore (0.4 m above chart datum) where the largest size particle fractions and lowest percentage organic content were found. Poisson distribution analysis showed the population was ‘clumped’ at the shore level, with a maximum number of 188 individuals in a 0.25 m2 quadrat, and within individual quadrats with up to 23 individuals in a 100 cm2 subsection. Sabella pavonina has an annual reproductive cycle which culminates in May/June spawning period. Oogenesis was a long process, first observed by the presence of small eggs (20 µm) from September onwards and maximum oocyte diameters were recorded (270 µm) between May and June prior to spawning. Between- and within-individual synchronicity was greatest in the early stages of oogenesis (October to February) when oocyte diameters ranged from 50–100 µm; however, increasing oocyte diameters were observed from March when rapid growth was initiated. Day length and sea temperature are candidates for exogenous cues controlling gametogenesis and possible mechanisms concerning fertilization in the species are discussed.