Although some conventional organic fibres for specialized uses have long been described as 'high modulus' fibres, for example high modulus polyester and rayon for car tyres, the level of modulus attained with organic fibres has been considerably less than that predicted theoretically or measured experimentally by means of x ray studies of the microcrystalline regions within fibres. Recently, however, there have appeared, mainly in the patent literature, descriptions of a range of novel organic fibres with moduli close to the levels predicted theoretically for organic polymers.