A method for measuring Mode I interlaminar fatigue crack growth rates under constant strain energy release rate conditions is described and results are presented for two graphite fiber composite materials. The test consists of a uniform width and thickness DCB specimen which is loaded in series with a linear elastic spring in order to provide optimum control of ΔG. Tests are run under displacement control, and delamination extension is determined by monitoring changes in compliance. A nonlinear elastic relationship between load and displacement was required due to a nonlinear material response caused by fiber bridging. Fatigue crack growth rates were observed to decline by as much as a factor of ten over a distance of less than 5 mm although this varied depending on material, lay-up, and temperature. The fatigue crack growth behavior is compared with delamination extension under quasi-static loading and noticeable differences explained in terms of the micromechanisms responsible.