The prevalence of intestinal parasitism in seminomadic pastoralists affected by river basin and irrigation developments is studied in relation to cultural and ecological factors. Five ethnic groups representing six cultural-ecological situations are studied in the Awash Valley of eastern Ethiopia. Sanitation level and other parasite transmission parameters in each of the six study populations are assessed by using a simplified semiquantitative system of scoring for variables. Results are examined to analyze the occurrence of infection in pastoralists largely continuing their traditional way of life and in tribesmen who settled in and around irrigation schemes and became farmers or farm laborers, and to evaluate some disease control measures. Prevalence of infection in the indigenous peoples is compared with that in migrant farm laborers from the Ethiopian highlands and the physical and cultural environment of the schemes and the Awash flood plains is examined to assess disease hazards created by the new farms and to make recommendations for parasitic disease control.