RESEARCH tive rodents were captured in weeds or roadsides inside or close to cultivated citrus or vegetables. Other rodents were captured in peridomestic sites associated with HPS cases and in woodbrush near one of the fincas. Focal concentra- tion of positive rodents appeared to occur, with multiple positive rodents often trapped in the same trap line. Characteristics of household or working habitats included a great deal of available potential food and cover for rodents attributable to substandard housing and sanita- tion. Sigmodontine rodents were also trapped in perido- mestic sites of the urban area (8 Has and 17 Has quarters), where the features of the environment and buildings were similar to suburban or rural areas. C. callosus was the only wild rodent species captured inside houses. This observa- tion is in accord with previous descriptions from Bolivia in relation to the Bolivian hemorrhagic fever outbreaks; C. callosus is the reservoir of Machupo virus, the arenavirus linked to Bolivian hemorrhagic fever, in the BHF-endem- ic area of El Beni (18). Control of the large Bolivian hem- orrhagic fever outbreaks of the 1960s was achieved through measures directed to prevent infestation of C. cal- losus in towns and villages. These same measures should also be useful in this area to prevent hantavirus transmis- sion, at least from rodents of the genus Calomys that are adapted to domestic and peridomestic settings. Our results favor the hypothesis that less virulent han- taviruses are responsible for the mild and subclinical ill- nesses circulating in this region. Ongoing investigations that include the genetic characterization of the viruses associated with the different clinical forms will help to clarify this point. 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