A memory of who knows what, so called transactive memory, can be an important cognitive structure in facilitating knowledge sharing in situations where successful collaboration depends on simultaneously maximizing sharing while mitigating its risks. We examine the development of transactive memory in cross- organizational networks—or ego-centric networks—that individuals build and maintain in their work. How do individuals develop transactive memory about who knows what in personally driven social networks that operate at the boundaries of cross-organizational work? In this paper, we advance a model of factors affecting the development of an individual’s transactive memory of his/her ego-centric work network and test the model with a group of professionals engaged in responding to unforeseen events related to national security. Overall, we find that frequent use of dialogic practices explain much of the degree to which an individual has developed a transactive memory of his/her ego-centric network. Dialogic practices are, in turn, affected by the degree to which the task is perceived as interdependent on the knowledge and actions of others and organizational support for learning. We note theoretical extensions to the literatures of transactive memory and information systems design for ego-centric networks.