Fatigue properties of two engineering rubbers have been measured in air and water. The fatigue crack growth rate, dc/dN, where c is the crack length and N the number of cycles, was measured as a function of tear energy for chloroprene rubber (CR) and natural rubber (NR). In general, the effect of water immersion on crack growth rates was relatively small. For NR, little effect of water immersion was seen and the fatigue threshold, which is the limit below which no mechano-oxidative fatigue growth will occur, was measured as 25 J/m(2) in both environments. For CR, a factor of two to three times lower crack growth rates was obtained in water compared to air, probably due to less influence of oxygen in water.