Abstract. Many brown howlers (Alouatta fusca) have died in a 3-month period in a subtropical forest in Southern Brazil. One was examined after a systemic illness. According to clinical signs, and necropsy and histopathology findings, yellow fever virus (YFV) infection was suspected. Tissue sections from liver, kidney, and lymphoid organs were screened by immunohistochemistry for YFV antigens. Cells within those tissues stained positively with a polyclonal antibody against YFV antigens (1:1,600 dilution), and yellow fever was diagnosed for the first time in the brown howler in the area.Yellow fever virus (YFV) infection is an acute arthropod-borne Flavivirus (family Togaviridae) infection that causes fever, jaundice, albuminuria, and hemorrhage. It occurs in 2 forms: urban and sylvan. There are 2 types of endemic areas: humid forests and emerging zones where urban and sylvan forms intermingle. 3 The vectors are Aedes aegypti in urban areas, 9 A. albopictus in suburban areas, 10 and tree-hole-breeding mosquitoes (Haemagogus spp.) in the forests. 9 The virus circulates in the forests in mosquito vectors causing scattered epizootics in nonimmune monkeys. Alternatively, transmission may be vertical from the female mosquito to her offspring, allowing virus survival from one rainy season to the next in A. aegypti eggs. 3 Yellow fever remains endemic in many regions of Africa and South America, despite the existence of an effective vaccine. 2,9 Currently, YFV infection may be confused with other similar hemorrhagic conditions. 2,4,9 In humans, severe necrotic lesions are seen at autopsy 2 in many organs, particularly the liver. Microscopically, YFV infection is characterized by midzonal necrosis of the liver with microvesicular fatty change