Bartonellosis, caused by Bartonella bacilliformis, is a clinically significant disease in parts of South America, where it is characterized by fever and hemolytic anemia during the often-fatal acute stage and warty skin eruptions during chronic disease. In this study, we evaluated owl monkeys (Aotus nancymaae) as a potential model for studying the immunogenicity and pathology of bartonellosis. Two groups of animals (n = 3 per group) received either 9.5 × 107 CFU B. bacilliformis by the ID route or 1.1 × 106 CFU by the IV route and were followed for 140 d. Animals were evaluated by physical exam, complete blood count or hematocrit (or both); infection was confirmed by Giemsa staining of blood smears, PCR amplification, and blood culture. On days 7 and 21, Giemsa-stained blood smears from both groups contained organisms (1% to 4% of erythrocytes). All blood cultures and PCR tests were negative. Complete blood counts and chemistry panels showed no difference from baseline. Serology revealed a greater than 4-fold increase in the IgM titer (compared with baseline levels) in the 3 animals from the ID group and 1 animal from the IV group. On day 35, a dermal lesion was excised from the inguinal region of 1 monkey from each group, with a second lesion excised on day 84 from the same monkey in the IV group. However the histopathology and immunostaining of these samples were not consistent with B. bacilliformis. The present study shows that owl monkeys can be infected with B. bacilliformis, but additional dosage studies are necessary to evaluate the usefulness of this species as a disease model for human bartonellosis. Copyright 2008 by the American Association for Laboratory Animal Science.