Oestrus ovis (Linné 1761) larvae are obligatory parasites of the nasal and sinus cavities of sheep and goats. Infestation is prevalent in hot and dry regions, such as Mediterranean countries. The current work was developed to establish the chronobiology of O. ovis in Sardinia, to determine the most suitable time for chemoprophylaxis. A survey was carried out during 1998, and sheep heads were collected monthly from local flocks. A total of 443 heads was examined, and the prevalence of oestrosis was 73.8%. We collected 2,691 larvae (mean = 6.07 ± 9.52), and the intensity was greatest in November. The humoral immune response against the nasal bot fly was analyzed by means of an indirect-ELISA using second-instar O. ovis excretory and secretory antigens. A seasonal variation in the antibody levels was observed, increasing from April and peaked in June and in September. A significant correlation was observed between first instar intensity and the mean relative humidity (r² = 0.120; P < 0.05), and between second-instar intensity and the mean temperature (r² = 0.241; P < 0.05). Three periods in the chronobiology of O. ovis were defined: diapause (October–February), the active phase of the endogenous cycle (March–September) and the exit phase (May–September). Our results showed that treatment in October–November was suitable, because first instars were in diapause, preventing the development of first into second instars, and second into third instars.