The nucleation of polypropylene on the surfaces of carbon and polyimide fibres was investigated by induction time measurement. The theory of induction time was used to analyse experimental results. It follows from this analysis that surfaces of the high-modulus carbon fibres generate nonepitaxial nucleation, whereas the high-strength carbon fibre and polyimide fibres generate epitaxial nucleation. On the basis of the induction time theory, quality of nucleation sites of the investigated fibres could be evaluated. It was found that epitaxial nucleation does not necessarily yield transcrystalline structure; instead separate epitaxially nucleated spherulites were observed on high-strength carbon fibre and on some polyimide fibres.