The behaviour and natural history of Megaloastia mainae, a longlegged salticid spider that appears to be endemic to northwest Australia, was investigated under natural conditions in the Kimberley Region of Western Australia. Megaloastia mainae is commonly found on rock escarpments, where it spends most of its time on downward-facing horizontal surfaces of large rocks. The diet of M. mainae comprises a variety of insects and spiders, including other salticids and web-building spiders. Megaloastia mainae builds large nests on the surface of the rock escarpment, which are generally occupied for at least several weeks. Contrary to previous reports, we found no evidence that M. mainae builds a prey-capture web or uses its nest as a predatory device. Megaloastia mainae is active during the day, and usually remains within 2 m of its nest. Megaloastia mainae orients to face prey from up to 1.2 m, and can move very quickly across the rock escarpment. Hunting is generally by slow stalking approaches, followed by rapid attacks. The elongated legs of M. mainae may be the result of selection for rapid locomotion in inverted environments. We also found that M. mainae that built nests close to conspecifics were more likely to be missing legs, suggesting that agonistic interactions might be an important source of injury for these spiders. [ABSTRACT FROM AUTHOR]