A seroepidemiologic study of California (CAL) serogroup viruses (genus bunyavirus, family Bunyaviridae) was carried out on 952 human and 1,834 animal sera collected from different ecological zones of Sri Lanka (latitudes 5-10 degrees N, longitudes 79-82 degrees E). The sera were screened for neutralizing antibody to Lumbo (LUM), snowshoe hare (SSH), and trivittatus (TVT) viruses by plaque neutralization tests on Vero cells. Of 2,786 sera screened, 262 (9.5%) had antibody to one or more viruses Twenty-two sera, selected to represent different species of origin and reaction profiles, were titrated against nine CAL viruses: LUM, SSH, TVT, Tahyna (TAH), California encephalitis (CE), La Crosse (LAC), Inkoo (INK), Melao (MEL), and Guaroa (GRO). Results suggested that there were multiple CAL viruses active in Sri Lanka, one or more of them closely related to LUMBO and SSH viruses, and another related to MELAO virus. These viruses were active in both the wet and dry zones of the country, and infected humans as well as a range of domestic livestock.