Continuous centrifugation and tangential flow ultrafiltration were used to size-fractionate and concentrate suspended particles from various turbid rivers of south-eastern Australia. These preparative steps produced highly concentrated samples, but the high load of suspended solids (up to 3.28 gl-1) did not interfere with the determination of phosphatase activity with an enzyme assay using p-nitrophenyl phosphate as the substrate. Phosphomonoesterase activity was an order of magnitude greater than phosphodiesterase activity; acid, neutral and alkaline phosphatases were detected. Most neutral (pH 7.5) and alkaline (pH 9.0) phosphatase activity was in the 0.2 - 1 mum size fraction: the next most active fractions were usually the 1-25 mum and the 100,000 daltons - 0.2 mum classes. Activity associated with free enzymes (i.e., that in the 10,000 - 100,000 daltons fraction) was usually low. Similar profiles were obtained with p-nitrophenyl phosphate and methylumbelliferyl phosphate substrates. Across the various size fractions within each river-water sample, phosphatase activity was usually more highly correlated with organic-matter content than with dry-matter content. Phosphatase activity was poorly related to chlorophyll-a contents. Phosphatase-producing bacteria were detected in all particulate fractions. The high activity in the 0.2 - 1 mum size fraction of turbid rivers might, therefore, be a function of either phosphatases associated with planktonic bacteria, or phosphatases adsorbed to the organic coating of suspended inorganic particles. In the coarser fractions, the additional contribution of attached bacteria cannot be excluded.