Abstract Microstructures of isotropie pyrolytic carbons have been examined by transmission electron microscopy, scanning electron microscopy and optical microscopy. Crystallite arrangements within the 0·5 μm dia growth features which make up these carbons differ greatly between high- and low-density carbons. Preferred orientations of crystallites are high in relatively large regions of the growth features of the high-density carbons while in the low-density carbons, the crystallites are in a tangled arrangement much as occurs in glassy carbons. The majority of the density defect in the low-density carbons is in the microporosity associated with this tangled structure and not in the large inter-growth-feature pores. Observations are made which suggest that the growth features are initially formed as free-floating particles before deposition on the substrate.