The Siamese Crocodile (Crocodylus siamensis) is critically endangered and, until a decade ago, few remaining wild populations were known to exist. Described here are the first in-depth surveys for C. siamensisin Laos with new field data on ecology and conservation. Small breeding populations of C. siamensisare confirmed to persist in Laos. During surveys between 2003 and 2008, C. siamensiswas recorded in 13 sites of six river systems, where at least 36 individuals (1–11 per site) were documented. In all sites, crocodile densities and recruitment rates were extremely low. Eight nests were recorded—among the first wild nests of C. siamensisto be reported. Perennial, thickly vegetated floodplain lakes are critical dry-season refugia and breeding habitats for C. siamensisin Laos. Opportunistic collection of crocodiles by local communities was observed, and at all sites there is increasing degradation of floodplain lakes for agriculture or economic development. National crocodile records were compiled and indicate that, historically, C. siamensiswas widespread in lowland riverine and palustrine habitats of Laos, with most records from Central and South Laos in the Mekong Plain. These records also suggest that a severe range decline has occurred over the past century, although most wetlands remain unsurveyed for crocodiles. Crocodylus siamensisis probably now extirpated from the Lao Mekong and many other wetlands. Remnant C. siamensispopulations in Laos are of global importance. All documented breeding sites, and most confirmed national records, are in rural lands outside the national protected area system, and conservation efforts will require community-based approaches.