The complexity of Salmonella transmission routes and the observation that Salmonella serovars diversify during evolution have led to the development of special methods for strain discrimination. Phage typing systems of Salmonella have been developed since the 30s (Marcuse, 1934), and still they are very helpful tools for epidemiological work (Rabsch, 1996). Nevertheless, their biological basis is not well understood. We have studied the Anderson typing system (1959) which has a long tradition, to answer the following questions: • To which extent do host controlled modification/ restriction systems interfere? • What is the significance of different phage receptor sites for typing? • How important are prophage controlled super-infection exclusion systems?