Recent electrophysiological studies in monkeys have implicated the prefrontal cortex (PFC) and posterior parietal cortex (PPC) in numerical judgments. The functional organization and respective contributions of these (and other) cortical areas, however, are unknown; their neural activity during numerical judgments has not been directly compared. We surveyed activity in the PPC and the anterior inferior temporal cortex while monkeys performed a visual numerosity judgment task and compared it with a population of PFC neurons. In the PPC, the proportion of numerosity-selective neurons was highest in the fundus of the intraparietal sulcus; only few numerosity-selective neurons were found in other PPC areas or the anterior inferior temporal cortex. Further, neurons in the fundus of the intraparietal sulcus responded and conveyed numerosity earlier than PFC neurons, suggesting that numerosity information flows from the PPC to the lateral PFC. This finding suggests a parieto-frontal network for numerosity in monkeys and establishes homologies between the monkey and human brain.