Corrosion fatigue is considered to be one of the most important factors in determining the life of static offshore structures such as platforms for oil and gas production; the combination of corrosive environment and cyclic stress can produce failure of metals by the development and growth of cracks. Seawater provides both the chemical reagent and, through wave action, the source of cyclic fatigue loading. Marine fouling can enhance both factors, first by enhancing the corrosive reactions and secondly by increasing the diameter and surface roughness of platform legs and bracing members. In relation to corrosion fatigue this enhancement is mainly due to the production of hydrogen sulphide by sulphate‐reducing bacteria. Both the effects of loading and hydrogen embrittlement can be independent of anti‐corrosion measures and thus need to be quantified and incorporated into the determination of the design life of the structure. Data are presented on the level of hydrogen sulphide that could be found under marine fouling and on corrosion fatigue crack growth in seawater containing various levels of both biologically produced and abiotic hydrogen sulphide. Crack growth rates are found to be enhanced, even at low levels, by hydrogen sulphide, and there are differences between biological and abiological environments.