The effect of a water environment on the near-threshold growth of delamination fatigue cracks was investigated with unidirectional laminates made from Ciba Geigy prepregs 914 C (T300/914) and from Toray prepregs P305 (T300/#2500). Tests were carried out under mode I opening loading using double cantilever beam specimens. The crack growth rate was correlated to the equivalent stress intensity range regardless of the stress ratio. For the 914 C laminate, the crack growth rate in water was about one tenth of that in air in the power-law region. For the P305 laminate, the effect of a water environment was small in the power-law region. Near the threshold, the crack grew slower in water than in air for both laminates. When the 914 C laminate specimen was moisture-conditioned, the growth rate in water was about 300 times faster than that of the specimen in water without conditioning. These environmental effects were explained on the bases of fractography and fracture mechanisms.