Abstract Waterflooding of the Rangely Oil Field of Colorado started in December 1957 in an effort to recover the maximum amount of oil from the reservoir. After 10 years of waterflooding, fluid production still exceeds fluid injection. As of October 1967 the deficit was 92 million barrels of fluid. A seismic study using seismograms from the Uinta Basin Seismological Observatory near Vernal, Utah, was made of the Rangely area. The epicenters of the earthquakes centered on the reservoir and extended beyond its limits. No prominent secondary area of activity was found. A study of the net fluid injected into the reservoir and the number of earthquakes that occurred disclosed no positive time correlation between these two parameters.