This paper presents a game theoretic model of entry. A potential entrant into a given market is postulated to be a currently produci ng firm, whose product is a substitute on the production side for the good being sold in the market. An entrant is capable of switching pr oduction between markets faster than firms in general can adjust thei r total outputs. To some extent, a market faced with entry as such wi ll resemble a contestable market, and limit pricing behavior will occ ur. Copyright 1988 by The London School of Economics and Political Science.