This thesis focuses on the emerging phenomenon of ecosystems. By means of a comparative case study of two mobile communication ecosystems it investigates the effects of governance choices by keystone companies on other ecosystem actors using a framework from the information systems literature. It provides examples of how governance choices have an influence on the adoption of the platform, fragmentation and the development of derivative platforms. It finds evidence that an open or closed governance theme applied by the keystone company creates several opportunities or challenges for other ecosystem actors. The findings can inform scholars on further directions of ecosystem research and give guidance to practitioners when thinking about the ecosystem strategy of their company and to make more conscious choices when companies join ecosystems.