An epidemic of Oropouche fever occurred in Santarém, Pará, Brazil in 1975. In the first survey for Oropouche antibodies involving a random sample of an entire city, infection rates varied from 0-44%, depending on the specific area within the city. Women had higher infection rates than men, but this difference was statistically significant only for persons older than 10 years of age. An analysis of school data showed that pupils in the evening classes had a greater increase in absenteeism during the epidemic period than those attending morning or afternoon classes. These data are compatible with the concept that Culicoides paraensis rather than Culex pipiens quinquefasciatus is the main vector of Oropouche virus in Brazil.