The speciation of Mn has been studied in Magela Creek, a tropical river in northern Australia containing water characterized by low pH, high temperature and extremely low ionic strength. Low concentrations (about 2-5�g L-1) of soluble Mn (i.e. filtration size <0.02�m) are typically found in these waters during the wet season, the main natural sources apparently being rainwater and groundwater. An additional source of soluble Mn has been identified, possibly being seepage from a mine retention pond. Rather surprisingly, the concentration of colloidal Mn (0.4-0.02�m) was almost three times higher than that of soluble Mn and was correlated with it. The primary process controlling the concentration of soluble Mn in this system appears to be rapid adsorption of soluble Mn to existing colloidal matter, followed by slower transfer of part of this surface-bound Mn to the interior of the colloid. Colloidal Mn is postulated to have a particularly long lifetime in this system because the removal process (aggregation to form particulate Mn) would be slow due to the extremely low concentrations of Ca and Mg ions during the wet season. A mixing experiment, undertaken to provide information on the possible fate of Mn in a mine retention pond if this wastewater should be discharged to the creek, clearly showed that Mn would be rapidly removed from the water column via colloidal and particulate forms. The precise mechanism was shown to be complex, depending upon the amount and characteristics of the colloidal and particulate matter present at the time of discharge, the changes in the Ca and Mg concentrations (which would influence the rate of aggregation), and the amount of turbulence in the creek.