Fast and slow muscle fibres were isolated from the axial muscles of conger eel (Conger conger L.) and examined by electron microscopy. Volume densities of mitochondria, VV (mit, f); intracellular lipid stores, VV (lip, f); and myofibrils, VV (mf, f) were respectively 0.23, 0.18, 0.47 for slow and 0.03, 0.002, 0.79 for fast fibres. Factors affecting the representative sampling of capillary supply were investigated using semi-thin (0.5 micron) sections. An estimate of the degree of capillary anisotropy was made using the assumption of a Fisher axial distribution (Mathieu, Cruz-Orive, Hoppeler & Weibel, 1983). The capillary bed is highly anisotropic. Mean capillary cross-sectional area, determined from electron micrographs, was 25.7 and 30.5 microms2 for slow and fast muscle, respectively. The number of capillaries per unit cross-sectional fibre area, NA (c,f); volume VV (c,f); and surface density, SV (c,f) of capillaries was 615 mm-1 0.016, 135 cm-1 for slow and 21.3 mm-1 0.0007, 4.7 cm-1 for fast muscle, respectively. The relationship between the extent of the capillary bed and mitochondrial volume density is discussed in relation to the function of different muscle types.