Malaria has reemerged as a significant public health disease threat in Peru, especially within the Amazon Basin region. This resurgence of human cases caused by infection with Plasmodium falciparum and Plasmodium vivax is thought to be associated with the spread of Anopheles darlingi, the principal South American malaria vector, into new areas of the Amazon Basin. However, comprehensive studies of the distribution for this species have not been conducted in Peru for several years, nor are historical accounts accurate enough to determine if An. darlingi was actually present and not collected or misidentified. Therefore, the objective of this study is to define the distribution of An. darlingi as well as obtain data on distribution and abundance of other Anopheles species in this region. Mosquitoes were collected during 2001 in the Departments of Loreto and Ucayali, the two largest Amazonian Departments of Peru. A total of 60,585 specimens representing 12 species of the subgenera Nyssorhynchus and Anopheles were collected at 82 (88.2%) of 93 collecting sites. The majority of mosquitoes obtained were identified as An. benarrochi, comprising 70.7% of mosquitoes collected, followed by An. darlingi (24.0%), Anopheles mattogrosensis (2.4%), and Anopheles triannulatus (1.5%). Anopheles darlingi was collected from 48.8% of sites, indicating that this species is established throughout central Loreto, including further west in the Amazon Basin than previously reported. These data suggest that this species is now found in areas of the Amazon Basin region where it has not been previously reported. Copyright © 2003 by the American Mosquito Control Association, Inc.