Eight new members of the phlebotomus fever arbovirus serogroup (family Bunyaviridae; genus Phlebovirus) from the Amazon region of Brazil are described. One serotype was recovered from a febrile patient, three from small wild animals and four from sand flies. A small serum survey carried out with the human isolate, Alenquer virus, suggests that it rarely infects man. Complement-fixation and plaque reduction neutralization tests were done, comparing the eight new viruses with other members of the phlebotomus fever serogroup. A close antigenic relationship was demonstrated between one of the new agents (Belterra) and Rift Valley fever virus. This finding is of considerable interest and deserves further investigation. Addition of these eight new viruses to the genus Phlebovirus brings to 14 the number of serotypes known to occur in the Amazon region and to 36 the total number reported worldwide. More detailed clinical and epidemiological studies should be conducted in Amazonia in order to define the public health impact caused by phleboviruses.