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We report new data about mayfly diversity from a 85 km2 area in East Kalimantan (Borneo, Indonesia). More than 40 mayfly genera have been collected in a lowland Dipterocarpaceae forest. They belong to the following families (by decreasing diversity order): Baetidae (12), Leptophlebiidae (7), Caenidae (6), Heptageniidae (5), Ephemerellidae (2), Potamanthidae (2), Teloganodidae (2), Isonychiidae (1), Neoephemeridae (1), Teloganellidae (1), Euthyplociidae (1) and Prosopistomatidae (1). The last family is recorded for the first time in Borneo. At least 10 taxa have not been assigned to known genera, especially within Baetidae and Caenidae. This incredible mayfly diversity is among the highest ever reported so far for an area of less than 10x10 km.