The effects of aging temperature and aging time on fatigue crack growth resistance have been studied for a 7475 (Al - Zn - Mg based) aluminium alloy. The alloy was tested in the underaged, peak aged, and overaged conditions after aging at 120 and 160 degreesC. Fatigue crack propagation tests were conducted in laboratory air using compact tension specimens in L-S orientation, under constant amplitude sinusoidal loading with an R ratio of zero. Results are discussed on the basis of resultant microstructures, fatigue crack growth rate diagrams and fractographic analysis. At 120 degreesC, a considerable effect of aging time on crack velocities at high stress intensities was seen. However, at 160 degreesC no significant dependency of crack growth rate on aging time was observed. The fatigue performance of overaged specimens was better for both aging temperatures. Also, lower aging temperature resulted in a more resistant structure against fatigue crack growth. Fractographic inspection showed that intermetallic particles play an important role in the crack growth behaviour of the Al - Zn - Mg alloy.