The article presents an empirical study of informal leadership in an international high-tech company. The empirical work consisted of qualitative observation and shadowing of managers in the company, and a total of 14 interviews. The transcribed fieldnotes and audio recordings of observations and interviews were analyzed thematically, resulting in three central themes: informality, information based authority, and information brokering. The general informal character of interactions at work included a network based information dissemination. In these networks, persons were percieved as informal leaders on the basis of their possession of reliable knowledge in technical as well as organizational domains. The informal leaders engaged in interpretation and brokering of information and knowledge, as well as in mediating strategic values and priorities on both formal and informal arenas. Informal leaders were thus seen to function on the level of the organization as a whole, and in cooperation with formal leaders. Drawing on existing theory of leadership in creative and professional contexts, this cooperation can be specified to concern task structuring. The informal leaders in our study contributed to task structuring through sensemaking activities, while formal leaders focused on aspects such as clarifying output expectations, providing feedback, project structure, and diversity.