A study on the epidemiology and seasonal dynamics of gastrointestinal nematode infections of sheep was carried out in a semi-arid region of eastern Ethiopia for 2.5 years (May 2003 to September 2005). The experimental flock comprised a total of 60 Black Head Ogaden sheep, consisting of four equal groups of young male and female and old male and female sheep. These grazed on communal pastures together with a larger university flock, as well as with animals owned by neighbouring small-holder farmers. A new experimental flock was established each year of the study. Parasitological data (EPG, faecal culture L3, PCV and FAMACHA estimates) and animal performance (weight change) were recorded each month on all experimental animals. In addition, four tracer lambs were assigned each month to the flock to determine the seasonal patterns of infective larvae acquired from pasture. Results showed distinct seasonal patterns associated with the bi-modal annual rainfall. High levels of infection occurred during the short and long rain seasons with peaks occurring in May and September of each year. Haemonchus contortus was the most prevalent parasite, followed by Trichostrongylus spp., with a number of other nematode species being occasionally recorded. H. contortus showed and increased propensity to undergo arrested development during the dry seasons. Correlations between EPG and PCV, EPG and FAMACHA eye scores, and PCV and FAMACHA eye scores were all highly significant (P<0.001). However, there was no significant association between the EPG and LW of the study animals during each study year. This information will provide a basis for developing epidemiologically based control strategies for gastrointestinal nematode parasites that are appropriate for flocks owned by small-holder farmers of semi-arid areas of eastern Ethiopia.