-The Pilbara region was surveyed for stygofauna between 2002 and 2005 with the aims of setting nature conservation priorities in relation to stygofauna, improving the understanding of factors affecting invertebrate stygofauna distribution and sampling yields, and providing a framework for assessing stygofauna species and community signifi cance in the environmental impact assessment process.Approximately 350 species of stygofauna were collected during the survey and extrapolation suggests that 500-550 actually occur in the Pilbara, although taxonomic resolution among some groups of stygofauna is poor and species richness is likely to have been substantially underestimated. More than 50 species were found in a single bore. Even though species richness was underestimated, it is clear that the Pilbara is a globally important region for stygofauna, supporting species densities greater than anywhere other than the Dinaric karst of Europe. This is in part because of a remarkable radiation of candonid ostracods in the Pilbara. Ostracods are the dominant stygofaunal group in terms of both species richness and animal abundance. Together, ostracods, copepods, amphipods and oligochates comprised 77% of species and 96% of animals collected.Stygofauna were found in 72% of samples collected and 81% of wells sampled. The average sample (including those without stygofauna) contained 3.2 ± 0.1 species. A feature of the Pilbara is that stygofauna occur across most of the landscape, often where the depth to groundwater is considerable, although yields were low where depth to groundwater was >30 m. Another feature is high endemicity: on the basis of current taxonomy 98% of the stygobites and 83% of the other groundwater species occur only within the region. Few factors affecting stygofauna occurrence could be identifi ed, however. Numbers of specimens and species collected were positively related to well diameter and negatively related to depth to groundwater. Numbers of species declined in inland sub-regions, although variability within sub-regions was high.While a range of freshwater chemistries occurred, 79% of water samples were weakly saline and NaCl dominated. Profi ling and purging of wells suggested that water quality measurements refl ected aquifer conditions in most situations. Water chemistry appeared to have limited infl uence on stygofauna occurrence in the Pilbara. Geology also appeared to have little effect on stygofauna occurrence but this may have been the result of non-random siting of wells; there was a bias towards wells being in transmissive locations that were not necessarily typical of the geology in which each well occurred.No potential reserves for stygofauna are recommended in this paper but nine areas of high stygofauna richness were identifi ed, including the listed Ethel Gorge stygofauna community. Theoretical analysis of species ranges suggested that half of the species found only in the vicinity of development projects will have ranges less than 680 km 2 . Consequently, projects involving ext...