The topography and chemoarchitecture of the striatum and pallidum in a monotreme, the short-beaked echidna (Tachyglossus aculeatus) have been studied using Nissl staining in conjunction with myelin staining, enzyme reactivity to acetylcholinesterase and NADPH diaphorase, and immunoreactivity to parvalbumin, calbindin, calretinin, tyrosine hydroxylase, neuropeptide Y, and neurofilament protein (SMI-32 antibody). All those components of the striatum and pallidum found in eutherian mammals could also be identified in the echidna's brain, with broad chemoarchitectural similarities to those regions in eutherian brains also apparent. There was a clear chemoarchitectural gradient visible with parvalbumin immunoreactivity of neurons and fibers, suggesting a subdivision of the echidna caudatoputamen into weakly reactive rostrodorsomedial and strongly reactive caudoventrolateral components. This may, in turn, relate to subdivision into associative versus sensorimotor CPu and reflect homology to the caudate and putamen of primates. Moreover, the chemoarchitecture of the echidna striatum suggested the presence of striosome-matrix architecture. The morphology of identified neuronal groups (i.e., parvalbumin, calbindin, and neuropeptide Y immunoreactive) in the echidna striatum and pallidum showed many similarities to those seen in eutherians, although the pattern of distribution of calbindin immunoreactive neurons was more uniform in the caudatoputamen of the echidna than in therians. These observations indicate that the same broad features of striatal and pallidal organization apply across all mammals and suggest that these common features may have arisen before the divergence of the monotreme and therian lineages.