Oestrosis, caused by the infestation of the sheep nasal bot Oestrus ovis larvae into the nasal cavities of sheep and goats, is a myiasis of worldwide distribution. The aim of this work is to study the prevalence and intensity of infection of O. ovis, as well as larval myiasis in mixed grazing sheep and goats, particularly due to Chrysomya bezziana, in Jazan Region, Saudi Arabia. A survey for larval myiasis due to O. ovis (Diptera: Oestridae) Linnaeus, 1761, in sheep and goats was conducted in Jazan municipal slaughterhouse, Jazan, Saudi Arabia. Out of 720 heads of slaughtered sheep and goats examined, 122 (16.94%) were found infected with O. ovis larvae, with a mean larval burden of 3.79 larvae per infected head. The mean infection rate of O. ovis in sheep was 30.84%, which is significantly higher than that in goats (3.78%) (P< 0. 05). Similarly, the mean infection rate of O. ovis in old sheep (62%) is significantly higher than in young sheep (7.5%) (P < 0.01). The results have shown that the infection rates of O. ovis among old and young goats were 6.47% and 1.5% respectively, which are not significantly different from one another (P>0.05). During this study, O. ovis larvae were collected throughout the year with varying infestation levels, indicating the existence of long favorable periods for larval development. Statistical analysis showed that there is no correlation between the intensity of infection with O. ovis (infection rates and larval burdens) and the prevailing climatic conditions (temperature and humidity). Larval myiasis due to other dipterous larvae in mixed grazing sheep and goats was investigated in 40 farms located in different ecological zones in Jazan Region. The larvae collected from infected sheep and goats were C. bezziana Villeneuve, 1914; Chrysomya albiceps (Wiedemann, 1819); Wohlfahrtia nuba (Wiedemann, 1830) and Sarcophaga africa (Wiedemann, 1824). The results showed that 20 farms (50%) were infected with larval myiasis. Similarly, it was found that 27.5%, 22.5%, 10% and 10% of the mixed grazing sheep and goats farms were infected with C. bezziana, C. albiceps, W. nuba and S. africa larvae, with the larval burden of 2.8, 2.6, 3.6 and 4.6 larvae per animal respectively.