Policymakers are increasingly using norm-based messages to influence individual decision-making. We partner with a metropolitan water utility to implement a natural field experiment examining the effect of such messages on residential water demand. The data, drawn from more than 100,000 households, indicate that social comparison messages had a greater influence on behavior than simple pro-social messages or technical information alone. Moreover, our data suggest social comparison messages are most effective among households identified as the least price sensitive: high-users. Yet the effectiveness of such messages wanes over time. Our results thus highlight important complementarities between pecuniary and non-pecuniary strategies.