As researchers have become more aware of the benefits of using fossilized insects to determine Quaternary climates, methods for their use have become more refined. However, pre-Quaternary insect fossils have not been utilized as indicators of climate, despite levels of preservation that are often equal to that of Quaternary fossils. Can pre-Quaternary insect fossils be reliable paleoenvironmental indicators? The Florissant Formation is well known for its exquisite compression fossils and diversity of insects. This study examines the Dipteran assemblage of the Florissant Formation to devise a method by which pre-Quaternary insect fossils can be used to determine paleoclimate. Because insects often have strict ecological tolerances, the overlap of climatic tolerances in an assemblage of insects may be used to determine past environmental conditions. The fossil Diptera from the Florissant Formation indicate a warm-temperate paleoenvironment and suggest that the ancient valley was at a low- to mid-elevation. A warm-temperate interpretation is in agreement with the fossil plant data from Florissant. However, the interpretation of low- to mid-elevation does not agree with many of the current studies based on plants, which indicate high paleoelevations for Florissant. The results of a taphonomic study to determine how biases in preservation could affect the climate results are also presented.