A census and an ecologic survey were performed in 39 villages of a rural district of Arsi Region, Ethiopia, in difficult field circumstances. Information on age, ethnic group, education and family relationship, as well as data on health facilities and availability of basic serivices were collected. Supervised students, working in teams, were used as interviewers. Communities were involved through plenary meetings and community health agents participated in the data collection process. A total of 64,714 people in 12,152 households were registered. The repeatibility of age assessment was investigated by comparing the results from two villages with data obtained in a pilot study carried out 6 months earlier. The technical error was only 0.80 and 1.67 in the 0-5 and 6-15 age-groups, respectively. Three percent of the total population was under one year, less than previously estimated. This may, in part, be due to the family planning programme in the region. Eighteen percent of the households were headed by females. School attendance was less common among females and in the Oromo ethnic group. The availability of basic services, including safe water and basic sanitation supplies, was very poor in the area.