The spatial patterns in diversity and abundance were remarkably consistent between locations in Darwin Harbour such that the species richness and abundance of polychaetes around Darwin Harbour appear reasonably predictable—with highest diversity and abundance in the seaward mangrove assemblage, decreasing progressively to landward, with few worms sampled in the hinterland margin. This distribution pattern presumably reflects the frequency of tidal inundation and the suitability of substrates—muds become increasingly moist and unconsolidated to seaward and the habitat opportunities for epifauna increase. In addition, frequency of tidal inundation directly influences recruitment such that the more frequently inundated seaward zones have a greater potential to receive larvae, and for larvae to survive. For example, species occurring in the tidal creek assemblage (∼ 1 m AHD) are inundated by 93% of annual tides, whereas those living in the tidal flat assemblage and above (> 2 m AHD) are only bathed by 58% of tides. High dry season evaporation rates in the tidal flat create hypersaline conditions which further add to the harshness of the mid-to upper-intertidal zone. Seasonal rainfall and higher tides during the wet season moderate the harsh environmental conditions in the tidal flat and the hinterland margin, which may contribute to the higher diversity and abundance observed in landward zones during the wet season. Desiccation and high salinity are likely to be important factors limiting worm populations in these habitats.