It is predicted that the market for position-based services is going to reach an annual turnover of billions of dollars in a few years. Besides this, also the game industry is estimated to meet a bright future, and there is a great interest already. This master’s thesis looks at a service that combines these two areas. The result is a position-based game called Mystery that is developed for the handheld computer Compaq iPaq 36-serie, which is well suited for this kind of game. To be able to locate a player we have used Ericsson’s Mobile Positioning System, MPS, which uses GSM positioning. The alternative would have been to use the Global Positioning System, GPS, but then the handheld computer would require extra hardware that is not available on the market yet. The prototype consists of a client and a server application. They are both developed in Java and they are communicating with each other through an http- protocol. One conclusion we have drawn form our work is that the kind of game we have developed requires higher accuracy than is achieved with GSM today, even though it is still possible to play the game. The disadvantage now is that the players have to move long distances, which is most likely if they are going by train, bus etc. A better accuracy would make it possible to shrink the distances so that it would also be remunerative to walk. When a better accuracy can be achieved a new possibility for this kind of game will open up.