A survey of 11 sites covering three large (> 10 km long, 6 sites) playa lakes and four pans (< 1 km(2), 5 sites) of the Yarra Yarra salt lake system in the Northern Agricultural Region of Western Australia commenced in 2001. These salt lakes are shallow and ephemeral, with inundation being more regular following winter rainfall, but summer inundation also occurred in 2001. Salinity was generally higher in playas (156-368 g l(-1)) than pans (30-284 g l(-1)), but salinity responded noticeably to heavy rainfall events, especially in pans. pH values in the playa lakes (6.68-7.82) were less variable than in the pans (6.81-8.08). The range of dissolved oxygen concentrations was greater in pans (3.7-14.4 mg l(-1)) than in playas (3.9-8.2 mg l(-1)). Cationic concentrations generally followed the pattern of sea water cation dominance. Benthic microbial communities comprised either cohesive to loosely mucilaginous mats, or thin films of diatoms. Five genera of diatom and two species of filamentous cyanobacteria were recorded.