Knowledge workers expend considerable effort managing fragmentation, characterized by constant switching among digital artifacts, when executing work activities. Activity-centric computing (ACC) systems attempt to address this problem by organizing activity-related artifacts together. But are ACC systems effective at reducing fragmentation? In this paper, we present a two-part study of workers using Lotus Activities, an ACC system deployed for over two years in a large company. First, we surveyed workers to understand the ecology of workplace tools they use for various tasks. Second, we interviewed 22 Lotus Activities users to investigate how this ACC tool fits amongst their ecology of existing collaboration tools and affects work fragmentation. Our results indicate that Lotus Activities works in concert with certain other tools to successfully ease fragmentation for a specific type of activity. We identify design characteristics that contribute to this result. © 2010 ACM.