Arguments regarding the high prevalence of interpersonal collectivism in Japan typically hinge on the assumption that Japanese communication networks are more enduring, frequently contacted, and dominated by kin and work ties than networks in Western countries. However, this assumption has not been examined using nationally representative data. Our analysis is based on core discussion network data collected from the 2003 Japanese General Social Survey and the 2004 American General Social Survey. Our results do not support this assumption in regards to frequency of contact nor dominance of kin and work ties. Nevertheless, there is some indication of high interpersonal collectivism in Japan insofar as Japanese core networks are somewhat more enduring than American core networks.