The cellular distribution of SPP1, which we recently identified as a gene with greater expression in the macaque primary motor cortex than in the premotor or prefrontal cortices, was examined in rhesus macaque, common marmoset, and rat brains. In situ hybridization histochemistry revealed that SPP1 mRNA was expressed specifically in pyramidal neurons in layer V of the sensorimotor cortex of the rhesus macaque. These SPP1 mRNA-positive neurons were most abundant in the primary motor area, followed by Brodmann area 5 and the supplementary motor area, in accordance with the distribution of corticospinal neurons. In addition, injection of a retrograde neuroanatomical tracer into the lateral corticospinal tract (CST) of the spinal cord caused labeling of SPP1 positive neurons, indicating the expression of SPP1 in corticospinal neurons. SPP1 was also expressed in the thalamus, brainstem, and spinal ventral horn of the rhesus macaque. Although SPP1 was also detected in the brainstem and spinal cord of the marmoset and the rat, it was not detected in their cerebral cortices. Selective expression in the corticospinal neurons of the sensorimotor cortex of the rhesus macaque suggests that SPP1 plays a critical role in the functional or structural specialization of highly developed corticospinal systems in certain primate species. J. Comp. Neurol. 518:2633-2644, 2010. (C)2010 Wiley-Liss, Inc.