PATENT RACES are important determinants of market structure. For a new firm, a patent can be an entry ticket into the market. For an firm already in the market, patents for related technologies can ensure that no new firm enters. In this spirit recent work has explored the implications of patenting for the persistence of monopoly (see, for example, Dasgupta [I982], Gilbert [I98I], Gilbert and Newbery [I982]). Much attention has been given to pre-emptive patenting''-the strategic acquisition of patents by an firm solely to prevent potential rivals from entering the market. In this context it is important to distinguish between two kinds of patent race. A standard race is a competition in which a prize is awarded to the first player to reach the finishing line. An asymmetrical race is a competition in which one player loses something of value if one of his rivals reaches the finishing line (he is content if nobody wins), but a prize is won by the first of the other players to reach the finishing line, if any of them does so. A patent race in which an firm's sole concern is to prevent potential rivals from entering his market is an asymmetrical race and not a standard race. The distinction between the two kinds of race can be captured only in models with a sufficiently rich dynamic structure. This, and other, advantages of a properly dynamic model are explained in section II. A model of an asymmetrical race with two firms is presented in section III. In this model the wins a prize, and the incumbent loses a prize, if the challenger is first to reach the finishing line. It is shown that the challenger is often automatically deterred from making any effort to win the race, because the strategic interactions between the players are such that the would outdo any reasonable effort made by the challenger in order to stop the challenger being first to reach the finishing line. Then the has no need to make any effort in the race in order to maintain his existing monopoly. The reason is that the incumbent-unlike the challenger-does not need to go all the way to the finishing line to achieve his objective. On the other hand, it may be truedepending on the parameters describing the firmsthat the challenger wins the patent, proceeding to the finishing line just as if he were the only contender for the patent. Of particular interest is the case in which the firms are identical