To study the time of onset of cases of Rocky Mountain spotted fever (RMSF) in relation to mean winter temperature, average daily solar radiation, elevation, and potential climax vegetation, we analyzed cases of RMSF from a 19-state area in the southeastern United States during the years 1970-1980 according to the counties in which patients resided. A bimodal incidence of RMSF, with peak onset of cases in mid-May and in mid-July, was noted in the oak-hickory-pine, oak-hickory, and Appalachian oak zones of potential climax vegetation during the years 1970-1974. In 1975-1980, however, coincident with an increase in incidence of RMSF, a single peak of illness in mid-June was observed in the oak-hickory-pine zone, where the number of cases of RMSF was highest; bimodality persisted in the oak-hickory and in the Appalachian oak zones. Analysis of cases in the zones in which bimodality persisted indicated that the first peak of illness may predominate in northern, cooler areas, and the second peak, in southern, warmer areas.