Constant amplitude fatigue crack propagation rate (da/dN) data are provided for a wide range of low and medium strength low alloy steels in the form of plate, bar and forgings. The ultimate tensile strength of the materials is less than 850 MN/sq m and their thicknesses range from 5 to 76 mm. The data have been derived from tests conducted in laboratory air at room temperature and mean da/dN curves are presented in terms of the stress intensity factor range and the 0.2 per cent proof stress of the material. Curves are drawn either side of the mean curve that indicate limits of scatter of 90 per cent of the data. The range of da/dN covered is from 10E-5 m/cycle to 10E-9 m/cycle. The effects of variations in mean stress, specimen size and thickness, and material yield strength are discussed. Details of all the materials and types of test specimen are tabulated and summary da/dN curves that compare the different groups of materials are given. A simple worked example illustrates how the data may be used in practice.