Retailers use sales "price promotions" for a number of potential reasons. There is relatively little research, however, on their strategic role among frequently consumed perishable products. Using a two-stage, nested logit model of retail equilibrium, we show that promotion will be most effective (ie. increase store-level sales) if products are highly differentiated, but stores are relatively similar. To test this hypothesis, we an oligopolistic model of promotion rivalry with category-level scanner data from the four largest supermarket retailers in a major U.S. metropolitan market. The results show that promotion has a greater impact on store share than product share, because the elasticity of substitution among stores is larger than the elasticity of substitution among products. Consequently, promotion has its greatest value in driving demand for differentiated products among stores that are similar. This finding supports the observed trend toward premium private label products being offered by supermarket retailers.