Koh Rong and Koh Rong Sanloem in Cambodia: how the islands differ, the best beaches, glowing plankton from July to October, how to get there from Sihanoukville and when to go.
You switch off the torch, wade knee-deep into warm black water, move your hand — and blue sparks flash around your fingers, as if the sea had been sprinkled with glowing powder. This is bioluminescent plankton, and on Cambodia’s islands it turns an ordinary night swim into a scene from a fantasy film. Koh Rong and its younger brother Koh Rong Sanloem are the country’s most accessible tropical island destination: white sand, turquoise water, beach bungalows and noticeably fewer tourists than on the Thai islands.
Koh Rong and Sanloem: what the difference is
Koh Rong is Cambodia’s second-largest island, part of the national marine park and popular with travellers who need beaches, bars and a choice of accommodation. Koh Rong Sanloem is the younger, smaller and noticeably quieter neighbour: people come here for real rest, excellent snorkelling and nights without city lights, in which the plankton is visible. If you want parties, choose Koh Rong; if you want silence and a starry sky — Sanloem.
Sanloem’s beaches
Saracen Bay on the east coast is the most developed: the main pier, resorts and restaurants are here, and the long beach suits beginners who need infrastructure. Lazy Beach and Sunset Beach on the west side of the island are more secluded — those who want quiet go there. M’Pai Bay in the north keeps the atmosphere of a fishing village and pleases with budget accommodation. The choice depends on what matters more to you — comfort or seclusion.
Glowing plankton
The most magical thing the islands offer is night swimming in bioluminescent plankton. The highest concentration of luminous organisms is found in dark zones of the coast not lit by lights: on the long beach of M’Pai Bay and in the far unlit corners of Saracen. When you swim or just stir the water, it flashes with neon-blue sparks. The best season is July to October: in these months dark moonless nights are most frequent and the concentration of plankton peaks. Darkness is essential for the effect: step away from lit bars, switch off your torch and wait two or three minutes for your eyes to adjust.
How to get there
You need to get to the islands from the port of Sihanoukville. Five speed ferry companies currently serve the route — Speed Ferry Cambodia, Island Speed Boat Cambodia, Buva Sea, Angkor Speed Ferry and GTVC — and speedboats take about 40–45 minutes to Koh Rong. It is better to check current prices and schedules on the carriers’ websites or on booking platforms like Camboticket and 12Go, because fares change depending on the season and type of vessel. In the rainy season sailings may be cancelled because of rough seas, so allow a spare day on the way back if you need to catch a flight.
When to go
The dry season, from November to April, is the most comfortable time for a beach holiday: the sea is calm, the sky is clear, ferries run on schedule. In the rainy season, from May to October, the islands are quieter and cheaper, but the weather is more capricious; the best time for plankton also falls in these months, so the compromise is quite reasonable if you are ready for a few overcast days.
Practical tips
Electricity on the islands is often limited: many bungalows run on generators, with light and air conditioning switched on only at night. Cash is needed in advance — there are few ATMs, and cards are not accepted everywhere. Take insect repellent, sunscreen and a waterproof case for your phone. Plastic on the islands is a serious problem, so take a reusable bottle and don’t leave rubbish on the beaches. Book accommodation in advance if you go at the weekend or in high season.
FAQ
How does Koh Rong differ from Koh Rong Sanloem? Koh Rong is bigger and livelier, Sanloem is quieter, calmer and better for snorkelling and watching plankton.
When is glowing plankton visible? Best from July to October, on dark moonless nights, away from lit beaches.
How do I get to the islands? By speedboat from the port of Sihanoukville; to Koh Rong it takes about 40–45 minutes in transit, and five companies serve the route.
Which beach should I choose on Sanloem? Saracen for infrastructure, Lazy and Sunset for seclusion, M’Pai for budget accommodation in a fishing village.
When is it best to go to the islands? In the dry season from November to April it is more comfortable, in the rainy season it is quieter and cheaper, and the plankton is better visible.
Prices, ferry schedules and accommodation conditions may change — check the current information before your trip.
