The HIV epidemic in Peru is concentrated primarily among men who have sex with men. HIV interventions have focused exclusively on a narrowly defined group of MSM and FSW to the exclusion of other populations potentially at increased risk. Interventions targeting MSM and FSW are insufficient and there is evidence that focusing prevention efforts solely on these populations may ignore others that do not fall directly into these categories. This paper describes non-traditional, vulnerable populations within low-income neighborhoods. These populations were identified through the use of ethnographic and epidemiologic formative research methods and the results are reported in this publication. Although the traditional vulnerable groups are still in need of prevention efforts, this study provides evidence of previously unrecognized populations at increased risk that should also receive attention from HIV/STI prevention programs. © 2007 Springer Science+Business Media, LLC.