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This study summarizes the current knowledge of the water beetle genus Hydraena Kugelann in Southeast Asia. The surprising species diversity and endemism rates in Hydraenopsis Janssens, the only subgenus present in Southeast Asia, are discussed. Data of five published Hydraena surveys from the Philippines and Singapore are used to evaluate species richness and the occurrence of species assemblages that are subject to the presence and quality of forests. Species richness was found to be generally higher in old grown forests. Some species appeared to be confined to near-natural forests. The denomination of indicator species is impeded by adequate sampling data, so it is suggested to intensify efforts in taxonomic and ecological research on these water beetles. It is concluded that the species richness and the common occurrence of the genus in the region might make Hydraena useful as biomonitoring organisms.