The endangered white-cheeked crested gibbon (Nomascus leucogenys), native to Laos, Vietnam, and perhaps China, remains little known and highly threatened. I studied seasonal variation in the diet, activity budget, and ranging behavior of three groups of white-cheeked crested gibbons in Nam Kading National Protected Area, Bolikhamxay Province, Laos, over 12 months in wet seasonal evergreen forest. Crested gibbons (Nomascus spp.) are speculated to be more folivorous than other gibbons, but this has never been confirmed because of the paucity of fieldwork on the genus. I studied diet in relation to forest seasonality to determine the contribution of leaves to the diet over an annual cycle. Although leaves were the main dietary item throughout the year (53-85% of monthly diet), gibbons substantially increased their consumption of fruit during periods when it was most abundant in the forest. Because fruit is a calorically rich source of food, gibbons seek fruit when it is abundant and obtained easily. Young leaf consumption increased when they did not have access to fruit, indicating that their diet is flexible but strongly dependent on seasonal availability of resources. In addition, rainfall had a negative association with fruit abundance and fruit in diet. Activity budgets are an important aspect of a species' ecology because they are directly related to home range use, energy allocation, and diet, but they have never before been studied in gibbons (Nomascus spp.) of the rainy, mountainous, forests of Laos. Annually, the three groups that I studied spent nearly equal amounts of time resting (30%), feeding (33%), and traveling (35%), but only a small amount of time singing (2%). However, the proportion of time allocated to different activities showed significant seasonal variation associated with rainfall and diet, and correlated with home range use. Gibbons increased traveling time and decreased feeding time when they ate more fruit, and they decreased traveling time and increased feeding time when they ate more leaves. When the gibbons spent more of their time traveling, they also had longer day range lengths, and used a higher percentage of their total home range. Moreover, when rainfall was high, the gibbons decreased traveling time and increased time resting and feeding. Average home range size was 37.9 hectares and daily average distance over which the gibbons ranged over the 12 month study was 1.48 km per day. Differences existed among the three groups. Overall, white-cheeked crested gibbons have a home range similar in size or larger than frugivorous gibbons, and larger than the folivorous siamang. Ranging was highly seasonal with shorter day ranges during times of low fruit availability and consumption. During times of high fruit availability and low rainfall, the gibbons took on an energy maximizing strategy where they maintained large home ranges, traveled longer distances and consumed larger quantities of fruit. Gibbons and their habitat in Laos have faced continuous threats over the past 10 years because of large scale development projects and subsistence hunting. Given the highly threatened status of the species in Vietnam and China, the Lao population is certainly the world's largest and the best hope for conservation of behavioral, ecological, and genetic diversity. Resources for conserving species in the country are very limited, and wildlife populations are already greatly fragmented. I describe the current conservation issues and based on the dietary and ranging information that I collected, recommend important conservation measures to safeguard the remaining populations of endangered gibbons from extinction.