It has been suggested that iron is a limiting factor on bloom-forming cyanobacteria in lake water. Although the availability of iron for phytoplankton depends significantly on its speciation, little is known about iron speciation in natural lake water. We investigated the horizontal distribution and temporal variation of dissolved iron and its chemical speciation in Lake Kasumigaura and its two inflowing rivers. Concentrations of dissolved iron and its organic ligands, determined by cathodic stripping voltammetry, clearly decreased as lake water flowed from the river entry points toward the center of the lake, indicating their riverine origin. The fraction of iron occurring in organic complexes tended to increase with the water flow. These results suggest that most of the dissolved iron in river water forms unstable soluble species, such as inorganic iron; thus, these unstable iron species may be scavenged in the mouths of rivers, and stable organic complexes of iron may flow to the center of the lake. Furthermore, most of the dissolved iron (88.2-99.9%) was present as organic complexes, and the inorganic iron level in Lake Kasumigaura (pFe' value=7.8-13.6) was similar to that observed in the open ocean. This result suggests that iron is an important factor determining the structure of the phytoplankton community in Lake Kasumigaura.