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 to examine 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 prosocial messages or technical information alone. Moreover, our data suggest that 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 nonpecuniary strategies.