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As climate change continues, sea turtle nests will be increasingly exposed to elevated incubation temperatures. Higher incubation temperatures influence many aspects of sea turtle development including sex determination and incubation length, but also survival. If temperatures in the nest increase above a thermal tolerance limit, then embryonic mortality may increase. The purpose of this research was to determine if there are differences in vulnerability to elevated temperatures across different stages of embryonic development and between loggerhead (Caretta caretta) and green (Chelonia mydas) sea turtles. Temperature dataloggers recorded nest temperature in the approximate center of loggerhead and green nests laid on the Boca Raton, Florida beach during the 2016 and 2017 nesting seasons. All unhatched eggs were collected from these nests following hatchling emergence. The eggs were dissected and the developmental stage at embryonic death was determined. The point of embryonic death was compared to the nest temperatures during that stage of the incubation period to determine if death corresponded to specific periods of elevated temperatures. Elevated nest temperatures increased embryonic mortality, but no developmental stage had higher mortality rates when exposed to any specific elevated temperatures compared to embryos that had not been exposed to that temperature. The most significant relationship was between mortality and the percent of time embryos were exposed to temperatures above 34 degrees C. This resulted in greater mortality of more developed embryos, as those embryos had a longer cumulative exposure to elevated temperatures. Loggerhead turtles exhibited higher rates of mortality compared to green turtles for almost all temperature exposure periods above 34 degrees C. Although few green nests reached 34 degrees C, green sea turtle embryos in south Florida may also have a higher thermal tolerance than loggerheads. Due to the increased embryonic mortality, and therefore, decreased hatching success, future management strategies may need to protect sea turtle nests from extended periods at elevated temperatures.