Records of 283 outbreaks of East Coast fever in Zimbabwe in the period 1914 to 1946 reveal that transmission of infection occurred throughout the year with peaks in January to March and May to July. The high level of transmission in January to March coincides with the known seasonal occurrence of adult Rhipicephalus appendiculatus. It is suggested that transmission at other times of the year may have been effected by a population of R. appendiculatus which did not exhibit a strictly seasonal life-cycle and has since been eradicated.