The callosal connections within the posterior parietal and occipital cortices were studied in the squirrel monkey with horseradish peroxidase tracing techniques. The data were evaluated with particular emphasis on the relationship of major callosal connections along the 17-18 border. The overall pattern of callosal connections in the squirrel monkey also was compared with callosal patterns in other New World simians. Our results show that the dense band of callosal connections along the 17-18 border in the squirrel monkey differs from the connections observed in other New World monkeys in that it is virtually confined to area 18 and avoids area 17. In addition to a continuous band of callosal connections in area 18 that parallels the 17-18 border, rostral extensions of the band are oriented perpendicular to the 17-18 border and present an obvious periodicity. The remaining parieto-occipital cortex contains a complex pattern of callosal connections that is strikingly similar to patterns reported for other New World monkeys. Thus, it is likely that the dorsolateral extrastriate visual cortex in the squirrel monkey is organized in a manner similar to that found within other New World monkeys.