Congregating fireflies are among the most marvellous organisms that fill the mangrove forests with “living fireworksâ€. These dainty creatures emit bioluminescence to attract mates by nightfall. Fireflies from the genus Pteroptyx aggregate on mangrove trees lighting up the riverine of several places in Malaysia, and this natural phenomenon provides local communities with lucrative business opportunity. Unsustainable management can bring disastrous consequences to the natural population of these graceful beetles. Without information on the current status of Pteroptyx fireflies in Klias Peninsula, any variation of this population in the future would be untraceable. This study was carried out to document the diversity of Pteroptyx fireflies and to identify their display trees in Klias Peninsula which is known as a firefly watching hotspot in Sabah. A total of four firefly species are found in Klias Peninsula, namely Pteroptyx tener Olivier, Pteroptyx malaccae Gorham, Pteroptyx valida Olivier, and Pteroptyx bearni Ballantyne. Pteroptyx tener is the dominant species in both Weston and Garama rivers while Pteroptyx bearni is the dominant species in Teratak River. The overall sex ratio on the display trees in Klias Peninsula was significantly biased towards the male. Seven mangrove species were selected as display trees by the congregating fireflies, particularly Excoecaria indica L. (Family Euphorbiaceae), Hibiscus tiliaceus, Nypa fruticans, Rhizophora apiculata, Avicennia alba, Excoecaria agallocha L., and Sonneratia alba J. Smith (Family Lythraceae)-IUCN Red List. Information on the display tree species provides indispensable information for conservation and rehabilitation purposes while information on the Pteroptyx firefly species composition and abundance can be used as reference to track the spatial and temporal variation of firefly communities.