Bioluminescent Plankton on Koh Rong

Last fact-checked: September 2026

On a dark night, every movement in the water around Koh Rong can light up in blue-green sparks. This natural light show is caused by bioluminescent plankton and is one of the main reasons many travelers come to the island. Here is what it is, when and where to see it, and how to enjoy it safely.

What makes the sea glow?

The glow comes from dinoflagellates, microscopic single-celled plankton that give off a short flash of light through a chemical reaction when the water around them is disturbed. Scientists believe the flash helps protect them from predators. The plankton is harmless to people: it does not sting or burn, and during the day you cannot see it at all.

When to go

The plankton is present around Koh Rong all year, but how bright it looks changes from night to night. Darkness matters most:

• New moon: The best nights are the few days around the new moon. The next new moons are on 10 October, 9 November and 9 December 2026 (Cambodian time).
• Time of night: Wait until it is completely dark, from about 7 pm. The darker the night, the better.
• Season: Some local guides report the strongest glow in the rainy season, when the water is richer in nutrients, while the dry season (November to April) brings calmer seas and more comfortable night swims. A few clear days without rain usually help.

Where to see it

The key is to get away from artificial light:

• Boat tours: The easiest option. Most evening boat tours from Koh Touch (typically $20-30, often with snorkeling, fishing and a barbecue) end with a swim in a dark bay.
• Long Set Beach: Walk away from the beach bars to a quieter stretch and wade into the shallow water.
• Lonely Beach: At the remote northern tip, with almost no light pollution, the glow can often be seen right from the shore.
• Koh Rong Sanloem: Saracen Bay and Lazy Beach on the neighboring island are also popular spots.

Safety tips

• Never swim alone at night and tell someone where you are going.
• Do not swim after drinking; there can be currents even when the sea looks calm.
• Bring a small torch for the walk to and from the beach, but switch it off in the water.
• Wear water shoes near rocks and coral and do not touch marine life.
• Keep your phone in a waterproof case, or better, leave it on the beach.

Photographing the plankton

The glow is faint and lasts only a split second, so a phone camera rarely captures what you see. You need a camera on a tripod, a long exposure and someone moving the water. Most people find it better to simply enjoy the moment.