Ecologic studies of Venezuelan encephalitis (VE) virus at a marsh habitat near the epicenter of the 1969 outbreak in Guatemala revealed that the virus was enzootic there. VE virus was isolated yearly during 1968–1973 from sentinel hamsters exposed during the rainy seasons, and from mosquitoes collected during July and August 1970. Hamsters yielded 41 strains of VE virus and virus was detected within 2 km of the edge of the marsh, in its interior, and at its western extreme 18 km from the central study site at La Avellana. One strain of virus came from a hamster that died in the dry season of January 1970. Culex mosquitoes yielded 20 strains of VE virus and Mansonia and Aedes one each. Culex (Melanoconion) and Aedes taeniorhynchus were most prevalent near the marsh. Hemagglutination-inhibition (HI) and neutralization antibody tests of sera showed that wild terrestrial mammals (opossums and rodents), humans, and dogs, but not wild birds, were frequently infected. Seven of 16 susceptible residents of villages at the edge of the marsh developed antibodies without symptoms during an 18-month period between September 1971 and February 1973. Only 1 of 5 sentinel rabbits, and none of 30 sentinel chickens developed VE HI antibody during August–September 1971, a period when virus activity was readily detected by the use of sentinel hamsters. Five strains of group C arbovirus (one identified as Nepuyo) were recovered from sentinel hamsters during 1968 to 1970, and one strain of Nepuyo virus was isolated from the blood of a person with a febrile illness during 1972. Two strains of Patois group arboviruses were isolated from Culex mosquitoes during 1970.