The Bolivian river dolphin or bufeo (Inia boliviensis) is one of four species of river dolphins in South America restricted to fresh water. Endemic to the Upper Madeira Basin in the Bolivian Amazon region, this species is categorized as "Vulnerable" by the Bolivian government. To determine the relative abundance of river dolphins during two hydrological seasons (rising and low water), we used the strip-width transect method in a 233-km segment of the Mamore River. We recorded the number and group size of river dolphins sighted in downstream and upstream transects on the same river. In addition, we examined river dolphin habitat preference in relation to six variables: (1) pH, (2) depth, (3) temperature, (4) habitat type, (5) shore type, and (6) visibility. Overall abundance was 0.52 dolphins/km across both seasons. Because recent density estimates of river dolphins have been fluctuating, it is difficult to determine a population trend. Moreover, we documented a significant habitat preference of dolphins for river confluences and meanders (p < 0.005), which affected the distribution and abundance of dolphins in this region.