Where to see bioluminescence or fireflies in Sri Lanka?

Bioluminescence (glowing plankton) can sometimes be seen in certain lagoons and beaches on dark nights, especially around Kalpitiya or some east coast spots, but it's not guaranteed and not organized as tours usually. Fireflies are more reliable in rural areas with lots of vegetation - some guesthouses in the hill country or near rivers organize short evening walks to see them. It's seasonal and weather-dependent. Ask locally when you arrive rather than booking specific tours in advance.