Olives grown in California may be infested by the olive fly, Bactrocera oleae (Gmelin)(Diptera: Tephritidae), an exotic pest discovered in the state around 1998. Despite continual oviposition by adults during the summer, immature stages may fail to survive during the hottest periods in the San Joaquin Valley, where temperatures often reach 40°C in July and August. Understanding the impacts of summer temperatures on olive fly mortality may prove useful for management strategies. Over a 5 day period, we investigated the effects of fluctuating temperatures (daytime highs: 35, 37.8, and 40.6°C; nightly low: 18.3°C) on olive fly eggs and instars within olive fruit. As individuals age, they can survive higher temperatures for longer periods. At 37.8ºC, 100% of the exposed eggs die after 1 day, whereas only about 90% of the first instars die after 4 days. Over 50% of second and third instars die only after exposure to 40.6 °C, although third instar mortality is made more complex by its prepupation behavior. Combined with GIS information on daily temperatures, information on temperature-induced immature mortality may be useful to growers in specific regions (e.g., Tulare and Fresno Counties, CA) wishing to reduce the frequency of insecticide bait applications during the summer months.