This paper theoretically considers a duopoly model in which all firms do not always employ personalized pricing. Our model incorporates the fact that firms engage in marginal cost-reducing activities after they decide whether to employ personalized pricing. When the ex ante cost difference between the firms is large, the less-efficient firm does not employ personalized pricing even when the fixed cost to do so is zero. This is because employing personalized pricing induces the rival firm to engage more in reducing its costs, which is more likely to harm the less-efficient firm.