Providing security for wireless sensor networks is a challenging task. Besides physical restrictions like limited computing abilities and available memory storage, a major problem results from the lack of any infrastructure components in a sensor network. Central components for security like e.g. key-distribution centers or Central Authorities (CA) are impossible to realize. It is therefore difficult to distribute encryption keys necessary for secure communication among sensors. This paper presents a novel approach to securely distribute keys to sensors joining the network. The approach is self organizing and minimizes memory consumptions as well as radio transmissions.