We have studied the cyto- and myeloarchitectural organisation of the spinal cord of an echidna (Tachyglossus aculeatus) with the aid of Nissl staining, darkfield examination and p-phenylenediamine staining. We have also examined the distribution of unmyelinated afferents by labelling with a peroxidase-conjugated lectin derived from Griffonia simplicifolia (B4 isolectin). The cytoarchitectural features characterising the laminar organisation of the spinal cord in eutherian mammals were broadly applicable to the spinal cord of this monotreme. In addition, we identified a distinct group of large neurons in the ventral part of lamina X, extending into the ventral funiculus, that we have called the median nuclear group. We were unable to identify a central cervical nucleus in this echidna on the basis of cytoarchitectural criteria, although all other spinal cord nuclei found in eutherians could be found in this monotreme. Lectin labelling with the Griffonia simplicifolia isolectin B4 revealed a subpopulation of dorsal root ganglion cells similar to those labelled in Eutheria. In this echidna, labelling of unmyelinated fibres was found in Lissauer''s zone and laminae I and II, as seen in rats (Rattus norvegicus); there were also deeper patches extending into laminae III to V and what appeared to be commissural axons approaching the dorsal grey commissure, which have not been seen in Eutheria. Fibre calibre in the dorsal and ventral roots of this echidna was similar to that reported in Eutheria, suggesting similar proportions of afferent fibre classes and α and γ motoneurons. In the echidna, mean diameter of myelinated dorsal root axons was 4.65 µm at T1 and 5.22 µm at L3, with a clear bimodal distribution in the L3 dorsal root showing distinct groups at 1 to 5 µm and 6 to 12 µm. These made up approximately 45 and 55% of the total myelinated axon population, respectively. Myelinated fibres in the ventral root at L3 showed two major peaks in distribution. These were at 1 to 4 µm (approximately 32% of the total myelinated fibre population) and at 7 to 14 µm (approximately 58% of the total myelinated fibre population). The cross-sectional area of the dorsal columns of this monotreme was comparable to that of a eutherian mammal of similar body weight, and myelinated axon calibre was similar to that seen in the domestic cat. Our findings indicated that spinal cord cytoarchitectural organisation is highly conserved across class Mammalia, although the lectin labelling findings suggested that termination of unmyelinated afferents in echidnas may differ from that found in Eutheria. The dorsal column system appears to be as anatomically well developed in this monotreme as in those eutherian mammals considered to have a pronounced discriminative tactil