1. The enzyme horseradish peroxidase (HRP) was injected into single Ib muscle afferent fibres in anaesthetized cats. Subsequently, histochemistry allowed the morphology of the axons and their collaterals in the lumbosacral spinal cord to be determined. 2. Eleven Ib axons were stained, seven from lateral gastrocneminus-soleus, one from medial gastrocnemius and three from muscles innervated by the posterior tibial nerve. Ten of the axons were traced into the dorsal roots and all but one (from the posterior tibial nerve) bifurcated upon entering the cord. Between 5.1 and 9.9 mm of each axon was stained and the fibres gave off eighty-four collaterals at intervals of 100-2300 micron, at an average spacing of about 900 micron. The spacing between collaterals on the (finer) descending axon branches was generally less than the intervals between collaterals on ascending branches. 3. All Ib collaterals had a characteristic morphology. The collaterals coursed cranially on a direct path through the dorsal horn to lamina IV or V before branching. They arborized widely in the intermediate region, mainly in lamina VI and in the dorsal part of lamina VII. Occasionally, less extensive arborizations were seen more dorsally in lamina IV and V. The rostro-caudal extent of individual collateral arborizations was limited to 200-400 micron and there was no overlap between adjacent collaterals. Each terminal arborization gave rise to 56-384 boutons, mainly of them 'en passant' type. 4. The results are discussed in relation to previous anatomical and electrophysiological studies.