On the back (west) side of the island is Koh Rong's finest beach, a 7km stretch of drop-dead-gorgeous white sand, dubbed Long Beach (also called Sok San Beach after the fishing village at its northern end). Longtail boats head here from Koh Tuch pier, depositing sun-seekers on the sand for a day of sunbathing and swimming. You can also walk here via a fairly rough trail through the jungle from Koh Tuch Beach (about 1½ hours).
Developers, not surprisingly, have their eye on this sensational sunset-facing strip, and a huge resort is in the works for the beach's south end. There is a village and a couple of places to stay at the northern end as well, but the rest of the beach is still virtually untouched.