During the period March 1996-July 1997, 417 heads of Awassi sheep slaughtered at the Irbid Abattoir (northern Jordan) were examined for the three larval instars (L1, L2 and L3) of Oestrus ovis. Of the 417 heads, 242 (58%) were infested with O. ovis larvae. Larval numbers were highly aggregated. The lowest number of larvae and the lower quartile were both zero, whilst the median was two and the upper quartile was 12. The highest number of larvae recovered from one head was 151. All three larval instars were observed in each month of the year. July and October had the highest proportions of L1, 75 and 78%, respectively, among infected animals (adjusted for age). The number of larvae increased with age. Infestation with live larvae was associated with inflammatory responses in the upper respiratory tract and with catarrhal or purulent discharge. The percentage of infested sheep and the mean monthly total number of larvae/sheep peaked in the warmer part of the year. Most larvae were L1 except during the spring when L2 and L3 predominated. Distribution analysis demonstrates that the numbers of larvae recovered in the sheep population followed a negative-binomial distribution. Furthermore, the negative-binomial constant k for each month correlated with the monthly prevalence.