Fifteen (6.4%) of 233 residents sampled in State Road Coulee (SRC) during 1972-73 had antibodies neutralizing La Crosse (LAC) virus, 9 (3.9%) trivittatus (TVT), 12 (5.2%) Jamestown Canyon (JC) and 2 (0.9%) Bunyamwera serogroup virus. Six of seven youths who had antibodies to LAC virus had been ill: three were serologically confirmed as cases of LAC encephalitis and three had possibly related illnesses. The other reported no illness. Possibly related illnesses were reported by 1 of 10 adults with antibody to LAC; also by 3 of 6 youths who had antibodies to TVT or JC viruses, and by 4 of 39 youths who did not have these antibodies. In prospective studies, 4 of 132 sampled again during 1977 had acquired antibodies, including 2 to LAC and 2 to JC virus. The LAC disease-infection rate was 50%. One of those with antibody to LAC virus was a girl with encephalitis who had a serologically confirmed infection a year after she had moved away from SRC. Her exposure to mosquitoes was not in SRC. The other was a boy who remained a resident but did not have encephalitis or other severe illness. The two who had acquired antibody to JC virus were boys who reported histories of undiagnosed febrile illnesses while attending summer camps in central Wisconsin. Control measures, including closure of treeholes, removing old tires and education, were applied. LAC infections in SRC were reduced from six during 1965 thru 1972, to only one during 1973 thru 1977. No cases have been found in SRC since 1977. Following expansion to a county-wide control program, LAC encephalitis in La Crosse County has been reduced from seven and eight cases during 1978 and 1979 to only one and two during 1980 and 1981. No cases have been found in La Crosse County so far during 1982.