This paper presents results from two large-scale natural field experiments that tested the effect of social norm messages on tax compliance. Using administrative data from >200,000 individuals in the United Kingdom, we show that including social norm messages in standard reminder letters increases payment rates for overdue tax. This result offers a rare example of social norm messages affecting tax compliance behavior in a real world setting. We find no evidence that loss framing is more effective than gain framing. Descriptive norms appear to be more effective than injunctive norms. Messages referring to public services or financial information also significantly increased payment rates. The field experiments accelerated the collection of tax revenue at little cost. •We present results from two large natural field experiments that use data from 200,000 individuals in the United Kingdom.•Including social norms and public services messages in official reminder letters increased payment rates for overdue tax.•This low-cost intervention offers a rare example of social norm messages increasing tax compliance in a real world setting.