Bioluminescent Plankton in Sihanoukville and Koh Rong

Part of the Sihanoukville and Koh Rong travel guide — see every topic in All Sihanoukville and Koh Rong Guides.

One of the most magical things on the islands happens after dark. Tiny sea creatures called plankton give off a blue-green light when the water moves. When you swim, splash or paddle, the water around you sparkles like stars. On Koh Rong and Koh Rong Sanloem there are few bright lights, so it is easy to see. This guide explains what the glow is, where to see it, and how to have the best night.

What makes the glow

The light comes from very small living things in the water, too small to see one by one. When the water moves around them, they give off a short flash of blue-green light. This is called bioluminescence. One flash is tiny, but millions of them together make the water shine. Every movement leaves a trail of light. If you move your hand slowly through the water, it looks like you are drawing with stars. The glow is natural and harmless. It is not dangerous to swim in, and it does not stay on your skin when you leave the water.

Where to see it

You can see the plankton from many beaches on both islands, as long as the beach is dark. On Koh Rong, Long Set Beach and the quieter west coast beaches are good choices. On Koh Rong Sanloem, many people see it in Saracen Bay and M'Pai Bay. Move away from bars, restaurants and resorts with bright lights. Walk to the darkest part of the beach, then wade into the water up to your waist. About one metre deep is ideal. The glow is stronger when you are fully in the water, so a night swim gives the best view.

When to go

The plankton can be seen on many nights of the year, but the glow changes from night to night. The best nights are dark ones, with no moon or only a thin moon. Around full moon, the bright moonlight makes the glow much harder to see. Check a moon calendar before you plan your trip. The best time of night is after the moon has set or before it rises, usually at least an hour after sunset. Clouds can help by blocking moonlight. Calm water is best. On rough nights, waves and stirred-up sand make it harder to see.

Night boat tours

Many guesthouses and tour shops offer evening boat trips. A typical trip includes a sunset view, sometimes fishing or a barbecue, and then a night swim in a dark bay to see the plankton. These trips are a good choice if your beach is too bright. Ask how many people go on the boat and whether there are life jackets. Some kayak and paddle board tours also go out at night. When you paddle, every stroke lights up the water. Night swims can be risky, so swim with others and stay near the boat or the shore.

Tips for a good night

Turn off your torch and phone light, and wait a few minutes for your eyes to get used to the dark. The longer you wait, the more you see. Photos are hard to take, as the light is very weak. Do not expect a phone picture to show what you see. Wear reef shoes if the sea floor is rocky. Do not swim alone or after drinking alcohol. Bring insect repellent for the beach, as mosquitoes and sand flies come out at night.

Bioluminescent Plankton at a glance

More in Sihanoukville & Koh Rong

All 6 Sihanoukville & Koh Rong guides·Cambodia tourist maps

Featured Videos