The Tierras Bajas is an area of 20,000 km2 of lowland deciduous forest in eastern Santa Cruz, Bo- livia, that has undergone rapid change during the past two decades. As part of the largest remaining area of intact deciduous tropicalforest in the world, it has been nominated a priority area for conservation by sev- eral environmental organizations. We quantified the spatial and temporal patterns of deforestation in the area by digital processing of high-resolution satellite imagery from 1975 through 1998. The estimated rate of deforestation was among the highest in the world for such a limited area, ranging from 160 km2/year in the earlyl980s to almost 1200 km2/year in the late 1990s. Although most deforestation up to 1984 was in Boliv- ian peasant and Mennonite colonies, most deforestation after 1984 was in non-Mennonite industrial soy- bean farms. The level of fragmentation of uncut forest, caused by the spatial patterns of deforestation, also differed among these broad land-use types. Deforestation in planned and spontaneous peasant colonies was complex in shape, forming relatively large areas of edge-affected forest, whereas that in Mennonite and other industrial farms was in large, rectangular increments, creating relatively less edge. But the distribution of these farms and the practice of initially clearing around the peripheries of properties resulted in the isolation of large areas offorest. In 1998four-fifths of the remaining forest were either within 1 km of a clearance edge or in isolated fragments of <50 km2. Compared with deforested areas, the areas of isolated and edge-affected forest were disproportionately large during the early stages offrontier colonization. These results imply that if the fragmentation effects of deforestation are to be minimized, conservation planning must occur at the ear- liest stages offrontier development.