The size of Rhipicephalus appendiculatus collected at different altitudes in the Eastern Province of Zambia between February 1985 and May 1986 and between October 1994 and December 1996 showed distinct variation dependent on altitude and season. The ticks were smallest during the dry season and at the start of the rains, and specimens were larger as the rainy season progressed. Second-generation adults where on average smaller than first-generation ticks. At higher altitudes, where a one-generation-per-annum phenology dominates, ticks were larger than at intermediate altitudes, where two generations per year are common. Larger size, associated with increased survival, is also favoured in low-lying, drier areas. Selective mortality of smaller adult ticks in years with a delayed rainy season appears to play an important role in the variation in size between years.