CONSIDERABLE effort has been directed toward analyzing the effect of entry on the behavior and performance of the firms supplying a market.1 Evidence has been presented suggesting that the entry of new firms varies directly with firm rivalry, typically measured by the amount of instability in the market shares of the leading firms.2 These studies, however, have considered entry in the conventional sense of a new firm gaining presence in a market. The impact of new brand entry by existing firms has been ignored. Evidently, it has been assumed that a new brand enters a market only when a new firm enters. Yet observation of consumer goods markets indicates that the intro- duction of new brands, and/or the repositioning of old brands by existing firms, is a frequently used form of non-price competition. Furthermore, the exclusive focus of previous investigators on firm market share instability, given a market environment dominated by multi-brand firms, may mask instability occurring at the brand level