313 sheep were examined in 1996 to assess the importance and seasonal evolution of Oestrus ovis infection in the Algerian region of El-Tarf. Prevalence was found to be 67.4%. The larval burden was 18 larvae by infected sheep. The prevalence was higher in older sheep than in lambs; intensity was similar. The different larval stages were found all along the year in sheep with prevalence ranging from 33.1 to 80.5% for L1, 9.7 to 43.9% for L2 and 8.4 to 23.0% for L3. The sheep were the least infected in winter (prevalence from 35.7 to 44% and intensity seven to ten larvae per sheep). The highest infection was found during the warm season (spring to autumn, prevalence from 62 to 90% and intensity ranging from 15 to 25). This larval evolution profile suggested the existence of one long cycle (November-April) and possibly two shorts cycles (May-October). This epidemiological pattern is similar to that in Morocco but was slightly different from the situation in Tunisia where the winter cycle was apparently of lesser importance.