To assess the population dynamics of drought-prone communities, we investigated 605 households in the pastoralist Boran community of Dubluk and in the agricultural community of Elka, both located in southern Ethiopia. The age and sex composition of the population as well as records of births, deaths and patterns of migration were observed for 2 consecutive years. Repeated surveys of the same households revealed much higher rates for deaths and births than did cross-sectional surveys with a one-year recall period. Indirect mortality estimates showed that the under 5 years mortality rates (per 1000 births) were 135 in Dubluk and 219 in Elka. Highest crude death rates were observed in Elka during periods of meningitis and malaria epidemics. During the period of observation, death rates fluctuated to a greater extent than birth rates. Both communities had very high rates of natural increase: in Dubluk 39·01000 and in Elka 37·11000. In Dubluk, this rate was far higher than any previously recorded and may have indicated that fertility regulating mechanisms, traditionally inherent in the pastoralist social organization, had become weaker as part of cultural changes. Dubluk represented a semi-nomadic society with a moderately high mobility pattern. Peak periods of migration coincided with times of food scarcity in Elka.