Potentially toxic cyanobacterial blooms are becoming common in the freshwater wetlands on the Swan Coastal Plain, Western Australia. During summer the dominant bloom-causing species belong to the genera Microcystis and Anabaena and to a lesser extent Aphanizomenon and Nodularia. Although toxic cyanobacteria have been recorded in the Swan–Canning and Peel–Harvey estuaries in Western Australia, very little is known about the blooms in the surrounding freshwater lakes. In this study, a total of 32 natural bloom samples representing 13 lakes were analyzed by HPLC for microcystin (MC)–LR, -RR, and -YR. Twenty-eight samples proved to be toxic. The highest total microcystin concentration ranged from 1645 to 8428.6 μg L−1, and the lowest concentrations were less than 10 μg L−1 with some below the detection limit (< 0.05 μg L−1). MC-LR (100%) was the predominant microcystin, followed by MC-YR (71.4%) and MC-RR (60.7%). The presence of a Nodularia spumigena bloom in the freshwater Lake Yangebup was associated with the detection of nodularins (1664 μg L−1). This is the first study to demonstrate the presence of microcystins and nodularins in urban lakes on the Swan Coastal Plain, Western Australia. © 2006 Wiley Periodicals, Inc. Environ Toxicol 21: 125–130, 2006.