The effect of a water environment on the growth of delamination fatigue cracks was investigated with unidirectional laminates made from ICI APC-2 thermoplastic prepregs (AS4/PEEK). Tests were carried out under mode I opening loading by using double cantilever beam specimens. The testing environment was water at 23°C and 50°C. Moisture conditioned specimens were also tested in water at 23°C. The water environment did not induce large detrimental effects. The crack growth rate was expressed as two power functions of the equivalent stress intensity range. The exponent of the power function in the low growth rate region was smaller than that in the high growth rate region. Lowering the frequency of stress cycling yielded accelerated crack growth in water, suggesting the time-dependent mechanism of crack growth. The difference of the environmental effects between CF/PEEK and CF/epoxy laminates was discussed on the basis of the fractographic observation and mechanism consideration.