Summary A fascicle of axons in an organotypic culture of rat dorsal root ganglion was analysed in 190 serial transverse sections, a total fascicle length calculated to be 12.8m. The number, position and continuity of microtubules and components of smooth endoplasmic reticulum (SER) were examined. The 187 microtubules traced in 27 axons showed a total of 32 terminations in the series; 14 of these were starts and 18 were stops in relation to progression away from the neuronal soma; three tubules began and ended in the series. Microtubules overall were long, their lengths calculated to average 108m. Terminations usually occurred close to the plasma membrane but otherwise exhibited no obvious morphological specialization. Microtubules in some axons were concentrated in the periphery of the axon, a region from which neurofilaments were excluded. The relative position of tubules in the axon did not change in any systematic way (for example, by rotation) throughout the series. The SER showed marked variation in form, from the expected membrane-bounded vesicle to profiles that resembled a microtubule or a thinner densely stained filament.