Caching is a key technique for improving data retrieval performance of mobile clients in mobile environments. The emergence of robust and reliable peer-to-peer (P2P) technologies now brings to reality what we call "cooperative caching" in which mobile clients can access data items from the cache in their neighboring peers. This paper considers a COoperative CAching scheme for mobile systems, called COCA. A cache signature scheme is devised for COCA that provides hints for the mobile clients to determine whether a required data item is cached by their neighboring peers based on their local state. The trade-off between the improvement in system performance and the overheads of the cache signature scheme in COCA is discussed. The performance of COCA with and without the cache signature scheme is evaluated through a number of simulated experiments. COCA is shown to be capable of effectively reducing the number of server requests and power consumption, as well as shortening the access latency as the number of neighboring peers increases. The inclusion of cache signature scheme further improves on the access latency.