[Truncated abstract] In my study of Australonereis ehlersi and Simplisetia aequisetis [Polychaeta: Nereididae] from the Swan River Estuary, Western Australia, I assessed the life history, geographical population structure and production of both species, then measured their roles in microbial denitrification and nitrogen cycling within the sediments of the estuary. Both species exhibit a mean life-span of approximately 1 year, a production:biomass turnover rate of about 3 and potentially are capable of reproducing throughout the year, peaking during winter to spring. A. ehlersi exhibited a marine euryhaline distribution, occurring only in the main basin and lower estuary, typically at a very low density of adults; S. aequisetis exhibited a euryhaline distribution, occurring estuary-wide during both summer and winter. High density and biomass of A. ehlersi occurred in the middle estuary (at Como), predominantly as winter- recruiting juveniles. Gravid, atokous adults spawned pelagically, with a 2 to 4 month larval development period preceding settlement. Intolerance of freshwater by the pelagic larvae possibly is the major reason excluding specimens from the upper reaches of the Estuary. Adult S. aequisetis brood eggs and embryonic larvae in tubiculous burrows; the life-cycle presumably progresses entirely in sediments of relatively stable interstitial salinity (compared to pelagic fluctuations), enabling recruitment by larvae and adults into the upper reaches of the Estuary. ... The ammonification rate was higher for A. ehlersi than S. aequisetis-inhabited cores, and lowest in uninhabited cores where polychaete excretion was absent. In the absence of C2H2, sediments of S. aequisetis inhabited cores indicated a lower net NH4+ influx than uninhabited cores, whereas A. ehlersi inhabited cores exhibited a slight net efflux of NH4+ from the sediment. The difference in magnitude of nitrogenous fluxes imparted by the two polychaete species is hypothesised to relate to the influence of their respective habits on the composition and activity of their associated sedimentary microbial community. Juvenile S. aequisetis are hypothesised to homogenise and aerate sediment continually, enhancing microbial nitrification and retarding anaerobic denitrification. Permanent A. ehlersi burrows would facilitate vertical and radial oxic/anoxic stratification of sediment which, combined with enhanced substrate supply through burrow ventilation, resulted in increased rates of microbial denitrification and nitrification. I have proposed a preliminary framework by which guilds of benthic fauna, each with similar designated habits, may be tested for predictable bioturbative influence on nitrogen cycling, i.e. whether particular habits may be considered 'functional groups'. In conclusion, the fine-scale effects of A. ehlersi and S. aequisetis on microbial nitrogen cycling are integrated with details of broader-scale population dynamics to define the role of polychaetes in estuarine nitrogen cycling, with a view to managing eutrophication.