In August 2002, two cases of hantavirus pulmonary syndrome (HPS) were confirmed in Mineros and Concepción, within the Santa Cruz Department of Bolivia. Extensive alteration of the native ecosystem, from dense forest to pasture or sugarcane, had occurred in both regions. An ecologic assessment of reservoir species associated with the human disease identified a single hantavirus antibody-positive Oligoryzomys microtis from Mineros and three hantavirus antibody-positive Calomys callosus from Concepćion. In Mineros, the virus from the O. microtis was 90% similar to sequences published for Río Mamoré virus. Viral nucleotide sequences from two C. callosus were 87-88% similar to the sequence of Laguna Negra virus. The viral sequence from the C. callosus was 99% identical to viral sequences obtained from the HPS patient in this area, implicating C. callosus as the host and Laguna Negra virus as the agent responsible for the HPS case near Concepción. Copyright © 2005 by The American Society of Tropical Medicine and Hygiene.